Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		$ 2,328,700,000	$ 2,746,000,000.0	$ 2,328,700,000	$ 2,328,700,000
Net realized and unrealized (losses) gains			(4,200,000)	280,600,000
Amortization/Accretion			(1,900,000)	(600,000)
Purchases			427,000,000.0	541,300,000
Sales			(510,000,000.0)	(667,000,000.0)
Transfers in			0	82,600,000
Transfers out			0	(82,600,000)
Ending balance	$ 2,656,900,000		2,656,900,000	2,483,000,000.0	2,746,000,000.0
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		842,600,000	780,000,000.0	842,600,000	842,600,000
Net realized and unrealized (losses) gains			(13,900,000)	115,200,000
Amortization/Accretion			0	100,000
Purchases			113,000,000.0	70,100,000
Sales			(214,000,000.0)	(343,500,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	665,100,000		665,100,000	684,500,000	780,000,000.0
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,160,500,000	1,109,600,000	1,160,500,000	1,160,500,000
Net realized and unrealized (losses) gains			3,600,000	62,000,000.0
Amortization/Accretion			(1,900,000)	(700,000)
Purchases			214,200,000	232,000,000.0
Sales			(231,900,000)	(319,700,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	1,093,600,000		1,093,600,000	1,134,100,000	1,109,600,000
NAV
Debt and Equity Securities, FV-NI [Line Items]
Transfers out				(71,700,000)
Common equity securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			100,000
Net realized and unrealized (losses) gains			0
Amortization/Accretion			0
Purchases			0
Sales			0
Transfers in			0
Transfers out			0
Ending balance	100,000		100,000		100,000
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	654,000,000.0	138,700,000	138,700,000
Net realized and unrealized (losses) gains			15,600,000	100,000,000.0
Amortization/Accretion			0	0
Purchases			78,600,000	136,700,000
Sales			(8,100,000)
Transfers in			0	82,600,000
Transfers out			0	0
Ending balance	740,100,000		740,100,000	458,000,000.0	654,000,000.0
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	202,300,000	186,900,000	186,900,000
Net realized and unrealized (losses) gains			(9,500,000)	3,400,000
Amortization/Accretion			0	0
Purchases			21,200,000	102,500,000
Sales			(56,000,000.0)	(3,800,000)
Transfers in			0	0
Transfers out			0	(82,600,000)
Ending balance	158,000,000.0		158,000,000.0	206,400,000	202,300,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	117,600,000		117,600,000	187,100,000	201,200,000	$ 214,200,000
Realized and unrealized losses			400,000	(200,000)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,947,200,000
Ending balance	2,774,500,000		2,774,500,000		2,947,200,000
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			969,400,000
Ending balance	779,600,000		779,600,000		969,400,000
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			1,121,400,000
Ending balance	1,096,700,000		1,096,700,000		1,121,400,000
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			654,100,000
Ending balance	740,200,000		740,200,000		654,100,000
Fair value measured on a recurring basis | Common equity securities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	464,700,000		464,700,000
Fair value measured on a recurring basis | Common equity securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	102,400,000		102,400,000
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	117,600,000		117,600,000		201,200,000
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	114,500,000		114,500,000		189,400,000
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	3,100,000		3,100,000		11,800,000
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
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 15,000,000.0	$ 114,700,000	$ 45,000,000.0	$ 114,700,000	$ 180,000,000.0