Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	$ 2,328,700,000	$ 3,208,300,000
Net realized and unrealized gains (losses)	280,600,000	(34,900,000)
Amortization/Accretion	(600,000)	(1,800,000)
Purchases	541,300,000	901,800,000
Sales	(667,000,000.0)	(1,798,000,000.0)
Transfers in	82,600,000	0
Transfers out	(82,600,000)	0
Ending balance	2,483,000,000.0	2,275,400,000
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	842,600,000	890,400,000
Net realized and unrealized gains (losses)	115,200,000	3,300,000
Amortization/Accretion	100,000	100,000
Purchases	70,100,000	315,700,000
Sales	(343,500,000)	(444,200,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	684,500,000	765,300,000
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,160,500,000	2,105,400,000
Net realized and unrealized gains (losses)	62,000,000.0	(34,600,000)
Amortization/Accretion	(700,000)	(1,900,000)
Purchases	232,000,000.0	539,000,000.0
Sales	(319,700,000)	(1,353,300,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	1,134,100,000	1,254,600,000
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138,700,000	77,200,000
Net realized and unrealized gains (losses)	100,000,000.0	(6,500,000)
Amortization/Accretion	0	0
Purchases	136,700,000	44,200,000
Sales	0
Transfers in	82,600,000	0
Transfers out	0	0
Ending balance	458,000,000.0	114,900,000
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	135,300,000
Net realized and unrealized gains (losses)	3,400,000	2,900,000
Amortization/Accretion	0	0
Purchases	102,500,000	2,900,000
Sales	(3,800,000)	(500,000)
Transfers in	0	0
Transfers out	(82,600,000)	0
Ending balance	206,400,000	140,600,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses	(200,000)	(600,000)
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
Ending balance	2,670,100,000
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
Ending balance	854,300,000
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,170,300,000
Ending balance	1,151,400,000
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138,700,000
Ending balance	458,000,000.0
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138,700,000
Ending balance	458,000,000.0
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
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
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
Ending balance	458,000,000.0
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	187,100,000		$ 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	169,800,000	$ 321,700,000	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	17,300,000		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
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	114,700,000
Kudu
Debt and Equity Securities, FV-NI [Line Items]
Transfers in	$ 71,700,000