Investment Securities (Net Realized and 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 investment (losses) gains, pre-tax	$ (156,700,000)	$ 245,600,000
Tax on Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(25,500,000)	52,200,000
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(131,200,000)	193,400,000
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Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains		67,500,000
Unrealized investment gain	30,000,000.0	114,700,000	$ 180,000,000.0
Foreign Currency Gain (Loss)
Debt and Equity Securities, FV-NI [Line Items]
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	$ 400,000	$ 300,000