Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			$ 3,208,300,000	$ 2,547,300,000
Net realized and unrealized gains (losses)			(34,900,000)	83,000,000
Amortization/Accretion			(1,800,000)	(4,700,000)
Purchases			901,800,000	1,516,400,000
Sales			(1,798,000,000)	(1,503,000,000)
Transfers in			0	11,100,000
Transfers out			0	(11,100,000)
Ending balance	$ 2,275,400,000	$ 2,633,800,000	2,275,400,000	2,633,800,000
Net realized and unrealized investment (losses) gains	10,500,000	33,700,000	(43,200,000)	70,000,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			890,400,000	279,500,000
Net realized and unrealized gains (losses)			3,300,000	30,400,000
Amortization/Accretion			100,000	0
Purchases			315,700,000	665,000,000
Sales			(444,200,000)	(243,900,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	765,300,000	731,000,000	765,300,000	731,000,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			2,105,400,000	2,093,800,000
Net realized and unrealized gains (losses)			(34,600,000)	39,000,000
Amortization/Accretion			(1,900,000)	(4,700,000)
Purchases			539,000,000	770,700,000
Sales			(1,353,300,000)	(1,255,000,000)
Transfers in			0	11,100,000
Transfers out			0	0
Ending balance	$ 1,254,600,000	1,649,700,000	$ 1,254,600,000	1,649,700,000
Number of Investments	1		1
Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			$ 0	0
Net realized and unrealized gains (losses)			0	100,000
Amortization/Accretion			0	0
Purchases			0	25,600,000
Sales			0	(500,000)
Transfers in			0	0
Transfers out			0	(11,100,000)
Ending balance	$ 0	14,100,000	0	14,100,000
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			87,200,000	91,400,000
Net realized and unrealized gains (losses)			(5,500,000)	(1,500,000)
Amortization/Accretion			0	0
Purchases			44,200,000	2,600,000
Sales			0	(2,000,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	125,900,000	90,500,000	125,900,000	90,500,000
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			125,300,000	82,600,000
Net realized and unrealized gains (losses)			1,900,000	15,000,000
Amortization/Accretion			0	0
Purchases			2,900,000	52,500,000
Sales			(500,000)	(1,600,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	129,600,000	148,500,000	129,600,000	148,500,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			600,000.0
Foreign Exchange Forward
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			3,500,000.0	13,000,000.0
Foreign Currency on Cash and Open Trades
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			4,200,000.0
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			3,259,100,000
Ending balance	2,467,500,000		2,467,500,000
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			1,041,400,000
Ending balance	1,079,900,000		1,079,900,000
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			2,130,500,000
Ending balance	1,261,700,000		1,261,700,000
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			87,200,000
Ending balance	125,900,000		125,900,000
Fair value measured on a recurring basis | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			2,129,700,000
Ending balance	1,108,700,000		1,108,700,000
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			296,500,000
Ending balance	121,100,000		121,100,000
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			1,833,200,000
Ending balance	987,600,000		987,600,000
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			87,200,000
Ending balance	125,900,000		125,900,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			0
Ending balance	0		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]
Beginning balance			0
Ending balance	0		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]
Beginning balance			87,200,000
Ending balance	125,900,000		125,900,000
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			176,100,000
Short-term investments, at amortized cost (which approximates fair value)	321,700,000		321,700,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			151,000,000
Ending balance	314,600,000		314,600,000
Short-term investments, at amortized cost (which approximates fair value)		71,600,000		71,600,000	$ 176,100,000	$ 175,000,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			25,100,000
Ending balance	7,100,000		7,100,000
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance			0
Ending balance	$ 0		$ 0
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		$ (12,500,000)		(12,500,000)	$ (3,700,000)	(1,200,000)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE				5,200,000
Star & Shield Insurance Exchange | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE				5,200,000
Star & Shield Insurance Exchange | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange | Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE				$ 0
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities debt reclassified to assets held-for-sale						6,600,000
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)						$ 100,000