Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 152,900,000	$ 35,200,000		$ (3,800,000)	$ 280,800,000
Trading Securities, Realized and Unrealized Gain (Loss)	2,200,000	200,000		(6,500,000)	12,400,000
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	30,000,000.0	16,900,000		32,200,000	38,000,000.0
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	0	300,000		400,000	300,000
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	98,300,000	27,800,000		(42,500,000)	139,100,000
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	24,600,000	(9,800,000)		6,100,000	103,400,000
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	15,000,000.0		$ 114,700,000	45,000,000.0	114,700,000	$ 180,000,000.0
Trading Securities, Realized and Unrealized Gain (Loss)					67,500,000
Investments Held [Member]
Debt and Equity Securities, FV-NI [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 150,700,000	$ 35,000,000.0		$ 2,700,000	$ 268,400,000