Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 3,208,300,000	$ 2,547,300,000
Total realized and unrealized gains (losses)	99,800,000	157,400,000
Amortization/Accretion	(2,500,000)	(9,100,000)
Purchases	1,394,300,000	3,332,500,000
Sales	(2,171,600,000)	(2,814,600,000)
Transfers in	0	18,700,000
Transfers out	0	(18,700,000)
Fair value investments	2,328,700,000	3,208,300,000	$ 2,547,300,000
Net realized and unrealized investment gains (losses), pre-tax	(108,300,000)	133,300,000	(27,400,000)
Net realized and unrealized investment gains	(108,300,000)	133,300,000	(27,400,000)
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	3,384,400,000
Fair value investments	2,542,900,000	3,384,400,000
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	2,129,700,000
Fair value investments	1,077,500,000	2,129,700,000
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	77,200,000
Fair value investments	138,700,000	77,200,000
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses), pre-tax	800,000.0	300,000
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 disclosure, investments	214,200,000	176,100,000	175,000,000
Foreign Currency on Cash and Open Trades
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses), pre-tax	4,200,000.0
Foreign Exchange Forward
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses), pre-tax	3,500,000.0	23,800,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	890,400,000	279,500,000
Total realized and unrealized gains (losses)	(64,900,000)	82,700,000
Amortization/Accretion	200,000	0
Purchases	514,700,000	1,209,300,000
Sales	(497,800,000)	(681,100,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	842,600,000	890,400,000	279,500,000
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	1,041,400,000
Fair value investments	1,047,000,000	1,041,400,000
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	296,500,000
Fair value investments	153,200,000	296,500,000
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	3,208,300,000
Fair value investments	0	3,208,300,000
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	151,000,000
Fair value investments	204,400,000	151,000,000
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,105,400,000	2,093,800,000
Total realized and unrealized gains (losses)	(64,400,000)	69,600,000
Amortization/Accretion	(2,700,000)	(9,100,000)
Purchases	783,800,000	2,007,900,000
Sales	(1,661,600,000)	(2,070,300,000)
Transfers in	0	18,700,000
Transfers out	0	0
Fair value investments	1,160,500,000	2,105,400,000	2,093,800,000
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	2,130,500,000
Fair value investments	1,170,300,000	2,130,500,000
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	1,833,200,000
Fair value investments	924,300,000	1,833,200,000
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	0
Fair value investments	0	0
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	25,100,000
Fair value investments	9,800,000	25,100,000
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	77,200,000
Fair value investments	138,700,000	77,200,000
Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0	0
Total realized and unrealized gains (losses)		0
Amortization/Accretion		0
Purchases		31,200,000
Sales		(12,500,000)
Transfers in		0
Transfers out		(18,700,000)
Fair value investments		$ 0	0
Number of investments | Investment		3
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	0
Fair value investments	0	$ 0
Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	77,200,000	91,400,000
Total realized and unrealized gains (losses)	16,200,000	(15,300,000)
Amortization/Accretion	0	0
Purchases	45,300,000	3,100,000
Sales	0	(2,000,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	138,700,000	77,200,000	91,400,000
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	77,200,000
Fair value investments	138,700,000	77,200,000
Level 3 Inputs | Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments		82,600,000
Total realized and unrealized gains (losses)		20,400,000
Amortization/Accretion		0
Purchases		81,000,000
Sales		(48,700,000)
Transfers in		0
Transfers out		0
Fair value investments			82,600,000
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 | Foreign Exchange Forward | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value disclosure, investments		(3,700,000)	(1,200,000)
Fair Value Measured at Net Asset Value Per Share | 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	135,300,000
Fair value investments		135,300,000
Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	135,300,000
Total realized and unrealized gains (losses)	13,300,000
Amortization/Accretion	0
Purchases	50,500,000
Sales	(12,200,000)
Transfers in	0
Transfers out	0
Fair value investments	$ 186,900,000	135,300,000
Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(5,200,000)
Star & Shield Insurance Exchange | Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Star & Shield Insurance Exchange | Level 1 Inputs | Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Short-term investments, at amortized cost (which approximates fair value)			100,000
Star & Shield Insurance Exchange | Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(5,200,000)
Star & Shield Insurance Exchange | Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Star & Shield Insurance Exchange | Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Star & Shield Insurance Exchange | Level 3 Inputs | Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		$ 0
Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fixed maturity investments held-for-sale			$ 6,600,000