Fair Value (Narrative) (Details) - USD ($)				3 Months Ended
	Apr. 30, 2021	Apr. 01, 2021	Mar. 31, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Common stock, par value (in dollars per share)			$ 0.001	$ 0.001		$ 0.001
Net gains on sale of OREO properties (less than)				$ 0	$ 0
Private equity investments carried at cost			$ 6,000,000	6,000,000		$ 8,000,000
Federal Reserve and Federal Home Loan Bank stock			109,000,000	109,000,000		109,000,000
Private equity investments accounted for under the equity method			68,000,000	68,000,000		$ 61,000,000
Fair Value, Nonrecurring
Derivatives, Fair Value [Line Items]
Gains (losses) from fair value changes (less than)				$ 1,000,000	1,000,000
Recursion Pharmaceuticals, Inc.
Derivatives, Fair Value [Line Items]
Carrying value of investment			$ 14,000,000
Common stock, par value (in dollars per share)			$ 6.71	$ 6.71
Subsequent Event
Derivatives, Fair Value [Line Items]
Lock-up period (in days)		180 days
Subsequent Event | Recursion Pharmaceuticals, Inc.
Derivatives, Fair Value [Line Items]
Public offering price (in dollars per share)		$ 18
Unrealized gain for investment	$ 56,000,000
IPO | Subsequent Event | Recursion Pharmaceuticals, Inc.
Derivatives, Fair Value [Line Items]
Shares held (in shares)		2,097,647
Level 3
Derivatives, Fair Value [Line Items]
Realized gains (losses) in the statement of income				$ (1,000,000)	15,000,000
Other real estate owned properties
Derivatives, Fair Value [Line Items]
Net gains on sale of OREO properties (less than)				1,000,000	1,000,000
Carrying value of OREO properties			$ 1,000,000	1,000,000	2,000,000
Impairments of OREO properties				$ 0	$ 0