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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	$ (131,200,000)	$ 193,400,000
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	2,200,000	21,100,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	400,000	0
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	(140,800,000)	111,300,000
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	(18,500,000)	113,200,000
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 30,000,000.0	$ 114,700,000	$ 180,000,000.0

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- Definition

The sum of the realized and unrealized net gain (loss) on trading securities during the period.

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- Definition

Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax

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