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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	$ 2,746,000,000.0	$ 2,328,700,000	$ 2,328,700,000
Net realized and unrealized (losses)	(157,100,000)	245,600,000
Amortization/Accretion	(800,000)	(300,000)
Purchases	232,900,000	217,600,000
Sales	(279,400,000)	(257,000,000.0)
Transfers in	0	10,900,000
Transfers out	0	(10,900,000)
Ending balance	2,541,600,000	2,534,600,000	2,746,000,000.0
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	780,000,000.0	842,600,000	842,600,000
Net realized and unrealized (losses)	(98,700,000)	91,700,000
Amortization/Accretion	0	100,000
Purchases	74,200,000	14,600,000
Sales	(109,900,000)	(149,700,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	645,600,000	799,300,000	780,000,000.0
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,109,600,000	1,160,500,000	1,160,500,000
Net realized and unrealized (losses)	(39,900,000)	40,700,000
Amortization/Accretion	(800,000)	(400,000)
Purchases	126,800,000	142,300,000
Sales	(105,400,000)	(105,400,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	1,090,300,000	1,237,700,000	1,109,600,000
Common equity securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	100,000
Net realized and unrealized (losses)	0
Amortization/Accretion	0
Purchases	0
Sales	0
Transfers in	0
Transfers out	0
Ending balance	100,000		100,000
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	654,000,000.0	138,700,000	138,700,000
Net realized and unrealized (losses)	(9,700,000)	114,000,000.0
Amortization/Accretion	0	0
Purchases	24,700,000	11,500,000
Sales	(8,100,000)
Transfers in	0	10,900,000
Transfers out	0	0
Ending balance	660,900,000	275,100,000	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	202,300,000	186,900,000	186,900,000
Net realized and unrealized (losses)	(8,800,000)	(800,000)
Amortization/Accretion	0	0
Purchases	7,200,000	49,200,000
Sales	(56,000,000.0)	(1,900,000)
Transfers in	0	0
Transfers out	0	(10,900,000)
Ending balance	144,700,000	222,500,000	202,300,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	137,600,000	269,800,000	201,200,000	$ 214,200,000
Realized and unrealized losses	(400,000)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	2,947,200,000
Ending balance	2,679,200,000		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	776,500,000		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	1,097,000,000.0		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	661,000,000.0		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	421,700,000
Fair value measured on a recurring basis | Common equity securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	101,900,000
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	137,600,000		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	130,900,000		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	6,700,000		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
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 30,000,000.0	$ 114,700,000	$ 180,000,000.0

X

- Definition

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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