Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		$ 2,328,700,000	$ 2,328,700,000	$ 3,208,300,000
Net realized and unrealized gains			347,500,000	31,300,000
Amortization/Accretion			(1,000,000.0)	(2,200,000)
Purchases			750,400,000	1,151,900,000
Sales			(856,200,000)	(2,024,200,000)
Transfers in			82,600,000	0
Transfers out			(82,600,000)	0
Ending balance	$ 2,569,400,000		2,569,400,000	2,365,100,000
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		842,600,000	842,600,000	890,400,000
Net realized and unrealized gains			121,800,000	51,900,000
Amortization/Accretion			200,000	100,000
Purchases			101,400,000	412,600,000
Sales			(362,300,000)	(457,600,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	703,700,000		703,700,000	897,400,000
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,160,500,000	1,160,500,000	2,105,400,000
Net realized and unrealized gains			66,600,000	(33,600,000)
Amortization/Accretion			(1,200,000)	(2,300,000)
Purchases			331,300,000	673,500,000
Sales			(465,200,000)	(1,565,000,000.0)
Transfers in			0	0
Transfers out			0	0
Ending balance	1,092,000,000.0		1,092,000,000.0	1,178,000,000.0
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000	77,200,000
Net realized and unrealized gains			154,200,000	(5,300,000)
Amortization/Accretion			0	0
Purchases			211,300,000	55,100,000
Sales			0
Transfers in			82,600,000	0
Transfers out			0	0
Ending balance	586,800,000		586,800,000	127,000,000.0
Hedge and private equity funds included in other long-term investments | NAV
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		186,900,000	186,900,000	135,300,000
Net realized and unrealized gains			4,900,000	18,300,000
Amortization/Accretion			0	0
Purchases			106,400,000	10,700,000
Sales			(28,700,000)	(1,600,000)
Transfers in			0	0
Transfers out			(82,600,000)	0
Ending balance	186,900,000		186,900,000	162,700,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	272,500,000		272,500,000		$ 214,200,000
Realized and unrealized losses			(300,000)	(700,000)
Foreign Exchange Forward
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses				(3,500,000)
Foreign Currency on Cash and Open Trades
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses				(4,200,000)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		2,542,900,000	2,542,900,000
Ending balance	2,841,900,000		2,841,900,000
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,047,000,000.0	1,047,000,000.0
Ending balance	964,700,000		964,700,000
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,170,300,000	1,170,300,000
Ending balance	1,103,500,000		1,103,500,000
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000
Ending balance	586,800,000		586,800,000
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000
Ending balance	586,800,000		586,800,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000
Ending balance	586,800,000		586,800,000
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	272,500,000		272,500,000		214,200,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	261,000,000.0		261,000,000.0	$ 313,700,000	204,400,000	$ 176,100,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	11,500,000		11,500,000		9,800,000
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	0		0		$ 0
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Derivative, fair value, net						$ (3,700,000)
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 35,300,000	$ 114,700,000	150,000,000.0
Kudu
Debt and Equity Securities, FV-NI [Line Items]
Purchases			70,100,000
Transfers in			$ 71,700,000