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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 2,328,700,000	$ 2,328,700,000	$ 3,208,300,000
Total realized and unrealized gains (losses)		433,000,000.0	(99,800,000)
Amortization/Accretion		(1,600,000)	(2,500,000)
Purchases		866,100,000	1,394,300,000
Sales		(950,300,000)	(2,171,600,000)
Transfers in		14,900,000	0
Transfers out		(14,900,000)	0
Fair value investments		2,746,000,000.0	2,328,700,000
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,542,900,000	2,542,900,000
Fair value investments		2,947,200,000	2,542,900,000
Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	138,700,000	138,700,000
Fair value investments		654,000,000.0	138,700,000
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Securities, realized and unrealized gain (loss)		200,000	800,000
Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value disclosure, investments				$ 176,100,000
Short-term investments, at amortized cost (which approximates fair value)		201,200,000	214,200,000
Foreign Currency on Cash and Open Trades
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Securities, realized and unrealized gain (loss)			4,200,000
Foreign Exchange Forward
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Securities, realized and unrealized gain (loss)			3,500,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	842,600,000	842,600,000	890,400,000
Total realized and unrealized gains (losses)		163,000,000.0	(64,900,000)
Amortization/Accretion		200,000	200,000
Purchases		165,300,000	514,700,000
Sales		(391,100,000)	(497,800,000)
Transfers in		0	0
Transfers out		0	0
Fair value investments		780,000,000.0	842,600,000
Level 1 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,047,000,000.0	1,047,000,000.0
Fair value investments		969,400,000	1,047,000,000.0
Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments		547,500,000
Level 1 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments		0
Level 1 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Short-term investments, at amortized cost (which approximates fair value)		189,400,000	204,400,000
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,160,500,000	1,160,500,000	2,105,400,000
Total realized and unrealized gains (losses)		76,100,000	(64,400,000)
Amortization/Accretion		(1,800,000)	(2,700,000)
Purchases		404,500,000	783,800,000
Sales		(529,700,000)	(1,661,600,000)
Transfers in		0	0
Transfers out		0	0
Fair value investments		1,109,600,000	1,160,500,000
Level 2 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,170,300,000	1,170,300,000
Fair value investments		1,121,400,000	1,170,300,000
Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments		136,300,000
Level 2 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0	0
Fair value investments		0	0
Level 2 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Short-term investments, at amortized cost (which approximates fair value)		11,800,000	9,800,000
Level 3 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	138,700,000	138,700,000
Fair value investments		654,100,000	138,700,000
Level 3 Inputs | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0	0
Total realized and unrealized gains (losses)		0
Amortization/Accretion		0
Purchases		100,000
Sales		0
Transfers in		0
Transfers out		0
Fair value investments		100,000	0
Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	138,700,000	138,700,000	77,200,000
Total realized and unrealized gains (losses)		181,200,000	16,200,000
Amortization/Accretion		0	0
Purchases		185,400,000	45,300,000
Sales		0
Transfers in		10,900,000	0
Transfers out		(4,000,000.0)	0
Fair value investments		654,000,000.0	138,700,000
Level 3 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	138,700,000	138,700,000
Fair value investments		654,000,000.0	138,700,000
Level 3 Inputs | Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Amortization/Accretion			0
Transfers in			0
Transfers out			0
Level 3 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Short-term investments, at amortized cost (which approximates fair value)		0	0
Level 3 Inputs | Foreign Exchange Forward | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value disclosure, investments			(3,700,000)
Fair Value Measured at Net Asset Value Per Share | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	186,900,000	186,900,000
Fair value investments			186,900,000
Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	186,900,000	186,900,000	135,300,000
Total realized and unrealized gains (losses)		12,700,000	13,300,000
Amortization/Accretion		0
Purchases		110,800,000	50,500,000
Sales		(29,500,000)	(12,200,000)
Transfers in		4,000,000.0
Transfers out		(10,900,000)
Fair value investments		202,300,000	$ 186,900,000
MediaAlpha
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Unrealized investment gain	$ 114,700,000	180,000,000.0
Kudu
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Kudu Transaction		70,100,000
Kudu | Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Kudu Transaction		0
Kudu | Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Kudu Transaction		0
Kudu | Level 3 Inputs | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Kudu Transaction		0
Kudu | Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Kudu Transaction		141,800,000
Kudu | Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Kudu Transaction		$ (71,700,000)

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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Amount of sale 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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This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis.

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