Rollforward of Fair Value Measurements by Level (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ 1,080,100,000	$ 1,128,500,000
Balance at January 1, 2017	5,039,700,000	4,050,900,000
Total realized and unrealized gains (losses)	(54,400,000)	29,600,000
Amortization/Accretion	(6,500,000)	(4,200,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	1,095,300,000	412,400,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	5,097,600,000	3,360,200,000
Other Investments	331,700,000		$ 323,300,000
Net realized and unrealized investment gains, after tax	44,100,000	21,000,000
Net realized and unrealized investment gains	51,300,000	29,500,000
Short-term investments, at amortized cost (which approximates fair value)	$ 230,900,000		287,000,000
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers	(5.00%)
Maximum price difference provided by pricing services	$ 1,000,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	197,100,000	852,900,000
Balance at January 1, 2017	612,500,000	1,152,200,000
Total realized and unrealized gains (losses)	22,400,000	11,300,000
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	124,500,000	109,000,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	562,300,000	419,600,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	866,900,000	272,600,000
Balance at January 1, 2017	4,118,500,000	2,531,400,000
Total realized and unrealized gains (losses)	25,500,000	19,100,000
Amortization/Accretion	(6,500,000)	(4,200,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	890,400,000	292,400,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	4,155,800,000	2,566,100,000
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Balance at January 1, 2017	0	70,000,000
Total realized and unrealized gains (losses)	200,000	500,000
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	58,100,000	0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	58,300,000	70,500,000
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Balance at January 1, 2017	0	0
Total realized and unrealized gains (losses)	0	0
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	0	0
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	2,000,000
Balance at January 1, 2017	177,700,000	169,500,000
Total realized and unrealized gains (losses)	(1,900,000)	1,100,000
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	200,000	2,100,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	174,000,000	172,700,000
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	14,100,000	3,000,000
Balance at January 1, 2017	131,000,000	127,800,000
Total realized and unrealized gains (losses)	8,200,000	(2,400,000)
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	22,100,000	8,900,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	147,200,000	131,300,000
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	5,195,900,000
Balance at March 31, 2017	5,181,300,000
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	887,000,000
Balance at March 31, 2017	790,200,000
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	4,131,200,000
Balance at March 31, 2017	4,158,800,000
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	177,700,000
Balance at March 31, 2017	232,300,000
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	4,256,800,000
Balance at March 31, 2017	4,175,400,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	269,400,000
Balance at March 31, 2017	101,400,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	3,987,400,000
Balance at March 31, 2017	4,015,700,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	58,300,000
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	177,700,000
Balance at March 31, 2017	174,000,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	177,700,000
Balance at March 31, 2017	174,000,000
Carrying value of investment accounted for using the equity method	3,000,000	3,700,000	3,500,000	$ 3,800,000
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	287,100,000
Balance at March 31, 2017	230,900,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	274,400,000
Balance at March 31, 2017	227,900,000
Short-term investments, at amortized cost (which approximates fair value)		211,300,000	287,100,000	261,700,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	12,700,000
Balance at March 31, 2017	3,000,000
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains	0	(8,800,000)
Star & Shield Insurance Exchange | Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments, at amortized cost (which approximates fair value)		300,000	100,000	100,000
Affordable Housing Development Fund
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	11,700,000	14,100,000	12,300,000	14,700,000
Affordable Housing Development Fund | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	11,700,000		12,300,000
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities Debt Reclassified to Assets Held-for-sale		$ 9,400,000	6,600,000	$ 9,500,000
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, fair value, net	(4,200,000.0)		$ (1,200,000.0)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	5,200,000
Star & Shield Insurance Exchange | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Star & Shield Insurance Exchange | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	5,200,000
Star & Shield Insurance Exchange | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Star & Shield Insurance Exchange | Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ 0