Estimated Fair Values (Roll-forward of Level 3 Assets and Liabilities) (Details) - Fair Value, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] - Available-for-Sale Securities, Private Label Residential Mortgage Backed Securities [Member] - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reconciliation of Available-For-Sale Securities Measured at Fair Value
Balance, beginning of year		$ 823	$ 1,053	$ 865	$ 1,104
Included in net impairment losses recognized in earnings	[1]	(2)	(1)	(3)	(1)
Included in other comprehensive income	[1]	(8)	(1)	(16)	(17)
Accretion of credit losses in net interest income	[1]	12	16	27	32
Settlements		(48)	(66)	(96)	(117)
Balance, end of year		$ 777	$ 1,001	$ 777	$ 1,001

[1] Related to available-for-sale securities held at period end.