Fair Value (Narrative) (Details) - USD ($)	3 Months Ended
	Dec. 31, 2019	Mar. 31, 2020
Fair Value Disclosure [Line Items]
Impairment loss on tax credit investments	$ 5,100,000
Securities purchased under agreements to resell outstanding	950,000,000.0
Level 2
Fair Value Disclosure [Line Items]
Securities purchased under agreements to resell outstanding		$ 0
Unsecured borrowings	4,500,000,000	4,500,000,000
Face amount	1,700,000,000	3,100,000,000
Level 3
Fair Value Disclosure [Line Items]
Face amount	$ 400,000,000	$ 600,000,000