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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment securities
Unrealized investment gains		$ 336,900,000	$ (83,600,000)	$ 97,500,000
Foreign Currency Transaction Gain (Loss), Unrealized		(300,000)	(11,800,000)	11,700,000
Net unrealized investment gains (losses), pre-tax		336,600,000	(95,400,000)	109,200,000
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments		(76,400,000)	9,100,000	(4,000,000.0)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax		0	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax		(76,400,000)	9,100,000	(4,000,000.0)
Unrealized investment gains after tax		260,500,000	(74,500,000)	93,500,000
Trading Securities, Unrealized Foreign Currenc Gain (Loss) Net of Tax		(300,000)	(11,800,000)	11,700,000
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax		260,200,000	(86,300,000)	105,200,000
Fixed maturity investments
Investment securities
Unrealized investment gains		38,600,000	(8,300,000)	13,800,000
Foreign Currency Transaction Gain (Loss), Unrealized		0	(14,800,000)	12,700,000
Net unrealized investment gains (losses), pre-tax		38,600,000	(23,100,000)	26,500,000
Common equity securities
Investment securities
Unrealized investment gains		109,800,000	(105,500,000)	99,300,000
Foreign Currency Transaction Gain (Loss), Unrealized		0	0	0
Net unrealized investment gains (losses), pre-tax		109,800,000	(105,500,000)	99,300,000
Other long-term investments
Investment securities
Unrealized investment gains		188,500,000	30,200,000	(15,600,000)
Foreign Currency Transaction Gain (Loss), Unrealized		(300,000)	3,000,000.0	(1,000,000.0)
Net unrealized investment gains (losses), pre-tax		188,200,000	$ 33,200,000	$ (16,600,000)
MediaAlpha
Investment securities
Unrealized investment gain	$ 114,700,000	$ 180,000,000.0

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Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement.

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