Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)	Jan. 01, 2017 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 2,547,300,000	$ 1,543,500,000
Total realized and unrealized gains (losses)	(157,400,000)	(26,200,000)
Amortization/Accretion	(9,100,000)	(6,100,000)
Purchases	3,332,500,000	4,728,300,000
Sales	(2,814,600,000)	(3,692,200,000)
Transfers in	18,700,000	70,000,000
Transfers out	(18,700,000)	(70,000,000)
Fair value investments	3,208,300,000	2,547,300,000	$ 1,543,500,000
Net realized and unrealized investment gains (losses)	133,300,000	(27,400,000)	260,500,000
Fixed maturity investments, at fair value		2,081,100,000
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,639,700,000
Fair value investments	3,259,100,000	2,639,700,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	279,500,000	789,000,000
Total realized and unrealized gains (losses)	82,700,000	2,000,000
Amortization/Accretion	0	100,000
Purchases	1,209,300,000	1,746,900,000
Sales	(681,100,000)	(2,258,500,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	890,400,000	279,500,000	789,000,000
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	441,700,000
Fair value investments	1,041,400,000	441,700,000
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,093,800,000	585,600,000
Total realized and unrealized gains (losses)	69,600,000	(8,000,000)
Amortization/Accretion	(9,100,000)	(6,200,000)
Purchases	2,007,900,000	2,868,800,000
Sales	(2,070,300,000)	(1,416,400,000)
Transfers in	18,700,000	70,000,000
Transfers out	0	0
Fair value investments	2,105,400,000	2,093,800,000	585,600,000
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	2,106,600,000
Fair value investments	2,130,500,000	2,106,600,000
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	91,400,000
Fair value investments	87,200,000	91,400,000
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)
Fixed maturity investments, at fair value		6,600,000.0
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 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)
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	2,087,700,000
Fair value investments	2,129,700,000	2,087,700,000
Fixed maturity investments | 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	101,500,000
Fair value investments	296,500,000	101,500,000
Fixed maturity investments | 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	1,986,200,000
Fair value investments	1,833,200,000	1,986,200,000
Fixed maturity investments | Level 3 Inputs
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	100,000
Amortization/Accretion	0	0
Purchases	31,200,000	69,900,000
Sales	(12,500,000)
Transfers in	0	0
Transfers out	18,700,000	(70,000,000)
Fair value investments	$ 0	$ 0	0
Number of investments | Investment	3	3
Fixed maturity investments | 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	$ 0
Fair value investments	0	$ 0
Fixed maturity investments | Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	6,600,000
Fair value investments		6,600,000
Fixed maturity investments, at fair value		6,600,000	9,500,000
Fixed maturity investments | Star & Shield Insurance Exchange | Level 3 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
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	91,400,000
Fair value investments	87,200,000	91,400,000
Other long-term investments | 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	0
Fair value investments	0	0
Other long-term investments | 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	0
Fair value investments	0	0
Other long-term investments | Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	91,400,000	103,600,000
Total realized and unrealized gains (losses)	(15,300,000)	(14,300,000)
Amortization/Accretion	0	0
Purchases	13,100,000	2,200,000
Sales	(2,000,000)	(100,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	87,200,000	91,400,000	103,600,000
Other long-term investments | 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	91,400,000
Fair value investments	87,200,000	91,400,000
Other long-term investments | Star & Shield Insurance Exchange | Level 3 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
Total hedge and private equity funds included in other long-term investments | Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	82,600,000	65,300,000
Total realized and unrealized gains (losses)	20,400,000	(6,000,000)
Amortization/Accretion	0	0
Purchases	71,000,000	40,500,000
Sales	(48,700,000)	(17,200,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	125,300,000	82,600,000	65,300,000
Total hedge and private equity funds included in other long-term investments | Star & Shield Insurance Exchange | Level 3 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
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses)	(300,000.0)
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	175,000,000	142,000,000
Fair value investments	176,100,000	175,000,000	142,000,000
Short-term investments | 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	162,300,000
Fair value investments	151,000,000	162,300,000
Short-term investments | 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	12,700,000
Fair value investments	25,100,000	12,700,000
Short-term investments | 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	0
Fair value investments	0	0
Short-term investments | Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	100,000
Fair value investments		100,000
Short-term investments | Star & Shield Insurance Exchange | Level 1 Inputs
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	$ 0	$ 100,000
Foreign Exchange Forward [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses)	(23,800,000.0)	(1,200,000.0)
Foreign Exchange Forward [Member] | 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	(1,200,000)
Fair value investments	$ (3,700,000)	$ (1,200,000)