Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($) Investment	Dec. 31, 2013 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 3,343,600,000	$ 3,889,000,000
Total realized and unrealized gains (losses)	(226,800,000)	83,300,000
Amortization/Accretion	(19,600,000)	(18,500,000)
Purchases	(2,377,100,000)	2,868,500,000
Sales	(2,210,100,000)	3,500,200,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	(394,500,000)	21,500,000
Fair Value, Assets Measured on Recurring Basis, Other	(61,400,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		0
Transfers in	41,800,000	53,000,000
Transfers out	(41,800,000)	(53,000,000)
Fair value investments	4,050,900,000	3,343,600,000	$ 3,889,000,000
Net realized and unrealized investment gains	225,400,000	78,500,000	133,900,000
Discontinued Operations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fixed maturity investments reclassified to assets held for sale (1)			(236,300,000)
Net realized and unrealized investment gains	15,100,000	205,400,000	27,800,000
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	3,720,400,000
Fair value investments	4,262,200,000	3,720,400,000
US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	105,200,000
Fair value investments	160,000,000	105,200,000
Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,422,000,000
Fair value investments	2,639,700,000	2,422,000,000
Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	309,900,000
Fair value investments	297,300,000	309,900,000
Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	376,800,000	310,400,000
Fair value investments	211,300,000	376,800,000	310,400,000
Debt securities issued by corporations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,171,700,000
Fair value investments	1,000,000,000	1,171,700,000
Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,171,700,000
Fair value investments	1,000,000,000	1,171,700,000
Mortgage-backed and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	82,200,000
Fair value investments	228,800,000	82,200,000
Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	10,800,000
Fair value investments	1,200,000	10,800,000
Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	84,200,000
Fair value investments	82,700,000	84,200,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	550,600,000	997,200,000
Total realized and unrealized gains (losses)	(263,000,000)	50,100,000
Amortization/Accretion	0	100,000
Purchases	(814,500,000)	886,900,000
Sales	(825,700,000)	1,384,200,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	(394,500,000)	(2,700,000)
Fair Value, Assets Measured on Recurring Basis, Other	(43,500,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		3,400,000
Transfers in	0	0
Transfers out	(1,200,000)	(200,000)
Fair value investments	1,152,200,000	550,600,000	997,200,000
Level 1 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	926,900,000
Fair value investments	1,363,500,000	926,900,000
Level 1 Inputs | US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,200,000
Fair value investments	133,400,000	51,200,000
Level 1 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,800,000
Fair value investments	134,000,000	51,800,000
Level 1 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	376,300,000
Fair value investments	211,300,000	376,300,000
Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	600,000
Fair value investments	600,000	600,000
Level 1 Inputs | Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,372,900,000	2,575,400,000
Total realized and unrealized gains (losses)	13,700,000	8,900,000
Amortization/Accretion	(19,600,000)	(18,700,000)
Purchases	(1,436,000,000)	1,805,300,000
Sales	(1,286,000,000)	2,065,900,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	24,200,000
Fair Value, Assets Measured on Recurring Basis, Other	0
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		(9,300,000)
Transfers in	41,800,000	53,000,000
Transfers out	0	0
Fair value investments	2,531,400,000	2,372,900,000	2,575,400,000
Level 2 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,373,400,000
Fair value investments	2,531,400,000	2,373,400,000
Level 2 Inputs | US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	54,000,000
Fair value investments	26,600,000	54,000,000
Level 2 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,293,800,000
Fair value investments	2,435,700,000	2,293,800,000
Level 2 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,700,000
Fair value investments	0	5,700,000
Level 2 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	500,000
Fair value investments	0	500,000
Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,166,400,000
Fair value investments	1,000,000,000	1,166,400,000
Level 2 Inputs | Mortgage-backed and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Level 2 Inputs | Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Level 2 Inputs | Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	82,200,000
Fair value investments	228,800,000	82,200,000
Level 2 Inputs | Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	10,200,000
Fair value investments	600,000	10,200,000
Level 2 Inputs | Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	13,100,000
Fair value investments	12,700,000	13,100,000
Level 3 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	420,100,000
Fair value investments	367,300,000	420,100,000
Level 3 Inputs | US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	76,400,000	81,800,000
Total realized and unrealized gains (losses)	1,100,000	1,800,000
Amortization/Accretion	0	100,000
Purchases	(35,300,000)	39,600,000
Sales	0	0
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	0
Fair Value, Assets Measured on Recurring Basis, Other	0
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		5,900,000
Transfers in	0	0
Transfers out	(40,600,000)	(52,800,000)
Fair value investments	$ 70,000,000	$ 76,400,000	81,800,000
Number of investments | Investment	6	6
Level 3 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 76,400,000
Fair value investments	70,000,000	$ 76,400,000
Level 3 Inputs | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	39,500,000	45,400,000
Total realized and unrealized gains (losses)	(7,800,000)	6,200,000
Amortization/Accretion	0	0
Purchases	0	5,000,000
Sales	(43,700,000)	17,100,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	0
Fair Value, Assets Measured on Recurring Basis, Other	(3,600,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		0
Transfers in	0	0
Transfers out	0	0
Fair value investments	0	39,500,000	45,400,000
Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	304,200,000	189,200,000
Total realized and unrealized gains (losses)	29,200,000	16,300,000
Amortization/Accretion	0	0
Purchases	(91,300,000)	131,700,000
Sales	(54,700,000)	33,000,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	0
Fair Value, Assets Measured on Recurring Basis, Other	(14,300,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		0
Transfers in	0	0
Transfers out	0	0
Fair value investments	297,300,000	304,200,000	189,200,000
Level 3 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	304,200,000
Fair value investments	297,300,000	304,200,000
Carrying value of investment accounted for using the equity method	3,800,000	5,200,000	$ 6,800,000
Level 3 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,300,000
Fair value investments	0	5,300,000
Level 3 Inputs | Mortgage-backed and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	71,100,000
Fair value investments	70,000,000	71,100,000
Prospector
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains	800,000	4,000,000
Consumer | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	116,000,000
Fair value investments	70,000,000	116,000,000
Consumer | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	259,200,000
Fair value investments	253,300,000	259,200,000
Consumer | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	115,900,000
Fair value investments	70,000,000	115,900,000
Consumer | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Consumer | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	100,000
Fair value investments	0	100,000
Consumer | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	259,200,000
Fair value investments	253,300,000	259,200,000
Consumer | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Consumer | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Financials | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	180,700,000
Fair value investments	694,700,000	180,700,000
Financials | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	254,700,000
Fair value investments	175,900,000	254,700,000
Financials | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	141,200,000
Fair value investments	694,700,000	141,200,000
Financials | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Financials | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Financials | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	254,700,000
Fair value investments	175,900,000	254,700,000
Financials | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	39,500,000
Fair value investments	0	39,500,000
Financials | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	74,400,000
Fair value investments	35,700,000	74,400,000
Health Care [Member] | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	181,800,000
Fair value investments	151,300,000	181,800,000
Health Care [Member] | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	74,400,000
Fair value investments	35,700,000	74,400,000
Health Care [Member] | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	181,800,000
Fair value investments	151,300,000	181,800,000
Health Care [Member] | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	57,100,000
Fair value investments	26,600,000	57,100,000
Industrial | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	107,300,000
Fair value investments	135,600,000	107,300,000
Industrial | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	57,100,000
Fair value investments	26,600,000	57,100,000
Industrial | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	102,000,000
Fair value investments	135,600,000	102,000,000
Industrial | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,300,000
Fair value investments	0	5,300,000
Utilities | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	18,100,000
Fair value investments		18,100,000
Utilities | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	65,600,000
Fair value investments	82,000,000	65,600,000
Utilities | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	18,100,000
Fair value investments		18,100,000
Utilities | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Utilities | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Utilities | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	65,600,000
Fair value investments	82,000,000	65,600,000
Utilities | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Utilities | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Energy | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,900,000
Fair value investments		4,900,000
Energy | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	84,100,000
Fair value investments	61,500,000	84,100,000
Energy | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,900,000
Fair value investments		4,900,000
Energy | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Energy | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Energy | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	84,100,000
Fair value investments	61,500,000	84,100,000
Energy | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Energy | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	44,800,000
Fair value investments	27,000,000	44,800,000
Technology | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	88,300,000
Fair value investments	60,000,000	88,300,000
Technology | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	44,800,000
Fair value investments	27,000,000	44,800,000
Technology | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	88,300,000
Fair value investments	60,000,000	88,300,000
Technology | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	28,500,000
Fair value investments	43,700,000	28,500,000
Communications | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	71,500,000
Fair value investments	49,200,000	71,500,000
Communications | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	28,500,000
Fair value investments	43,700,000	28,500,000
Communications | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	71,500,000
Fair value investments	49,200,000	71,500,000
Communications | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Materials | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	12,000,000
Fair value investments		12,000,000
Materials | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,700,000
Fair value investments	31,200,000	51,700,000
Materials | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	12,000,000
Fair value investments		12,000,000
Materials | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Materials | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Materials | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,700,000
Fair value investments	31,200,000	51,700,000
Materials | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Materials | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Other | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	75,200,000
Fair value investments	74,400,000	75,200,000
Other | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	7,500,000
Fair value investments	0	7,500,000
Other | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,900,000
Fair value investments	0	1,900,000
Other | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Other | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	73,300,000
Fair value investments	74,400,000	73,300,000
Other | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	7,500,000
Fair value investments	0	7,500,000
Other | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Other | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Exchange Traded Funds [Member] | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	141,800,000
Exchange Traded Funds [Member] | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	120,500,000
Exchange Traded Funds [Member] | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	21,300,000
Exchange Traded Funds [Member] | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Common equity securities | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	611,700,000
Fair value investments	1,113,900,000	611,700,000
Common equity securities | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	498,800,000
Fair value investments	1,018,200,000	498,800,000
Common equity securities | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	73,400,000
Fair value investments	95,700,000	73,400,000
Common equity securities | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	39,500,000
Fair value investments	0	39,500,000
Symetra
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total realized and unrealized gains (losses)	(258,800,000.0)
Affordable Housing Development Fund [Member] | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	16,800,000
Fair value investments	$ 14,700,000	$ 16,800,000