Investment Securities (Net Realized and 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]
Trading Securities, Realized and Unrealized Gain (Loss)	$ 2,200,000	$ 200,000		$ (6,500,000)	$ 12,400,000
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Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized and Unrealized Gain (Loss)					67,500,000
Unrealized investment gain	15,000,000.0		$ 114,700,000	$ 45,000,000.0	$ 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