Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 48,200,000		$ 76,400,000	$ 257,800,000	$ 48,400,000
Net Foreign Exchange Gains, pre-tax	(800,000)		(100,000)	(800,000)	(11,600,000)
Net unrealized investment gains (losses), pre-tax	47,400,000		76,300,000	257,000,000.0	36,800,000
Net Unrealized Gains (Losses), tax	13,000,000.0		12,000,000.0	(60,500,000)	17,100,000
Net Foreign Exchange Gains, tax	0		0	0	0
Total Net Unrealized Gains (Losses) Reflected in Earnings, tax	13,000,000.0		12,000,000.0	(60,500,000)	17,100,000
Net Unrealized Gains (Losses), after-tax	35,200,000		64,400,000	197,300,000	31,300,000
Net Foreign Exchange Gains, after-tax	(800,000)		(100,000)	(800,000)	(11,600,000)
Total Net Unrealized Gains (Losses) Reflected in Earnings, after-tax	34,400,000		64,300,000	196,500,000	19,700,000
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	7,500,000		1,500,000	43,400,000	(15,700,000)
Net Foreign Exchange Gains, pre-tax	0		0	0	(14,800,000)
Net unrealized investment gains (losses), pre-tax	7,500,000		1,500,000	43,400,000	(30,500,000)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	0		(100,000)	200,000	(100,000)
Net Foreign Exchange Gains, pre-tax	0		0	0	0
Net unrealized investment gains (losses), pre-tax	0		(100,000)	200,000	(100,000)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(11,600,000)		54,900,000	60,100,000	43,700,000
Net Foreign Exchange Gains, pre-tax	0		0	0	0
Net unrealized investment gains (losses), pre-tax	(11,600,000)		54,900,000	60,100,000	43,700,000
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	52,300,000		20,100,000	154,100,000	20,500,000
Net Foreign Exchange Gains, pre-tax	(800,000)		(100,000)	(800,000)	3,200,000
Net unrealized investment gains (losses), pre-tax	51,500,000		$ 20,000,000.0	153,300,000	$ 23,700,000
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 35,300,000	$ 114,700,000		$ 150,000,000.0