Other Comprehensive Income (Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, before Tax [Abstract]
Increase in net unrealized gains (losses) on securities, pre-tax	$ 2,113	$ 1,529	$ (536)
Less: Net realized gains (losses) reclassified to earnings, pre-tax	302	40	4
Net increase (decrease), pre-tax	1,811	1,489	(540)
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax [Abstract]
Increase in net unrealized gains (losses) on securities, tax effect	(485)	(351)	123
Less: Net realized gains (losses) reclassified to earnings, tax effect	(69)	(9)	(1)
Effect of income taxes	(416)	(342)	124
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, After Reclassification Adjustments, after Tax [Abstract]
Increase in net unrealized gains (losses) on securities, after tax	1,628	1,178	(413)
Less: Net realized gains (losses) reclassified to earnings, after tax	233	31	3
Net increase (decrease), after-tax	1,395	1,147	(416)
Net unrealized gains (losses) on cash flow hedge derivatives [Abstract]
Increase in net unrealized gains (losses) on cash flow hedges, pre-tax	918	334	(118)
Less: Net realized gains (losses) reclassified to earnings, pre-tax	421	37	60
Net increase (decrease), pre-tax	497	297	(178)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, Tax [Abstract]
Increase in net unrealized gains (losses) on cash flow hedges, tax effect	(211)	(77)	27
Less: Net realized gains (losses) reclassified to earnings, tax effect	(97)	(9)	(14)
Effect of income taxes	(114)	(68)	41
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Increase in net unrealized gains (losses) on cash flow hedges, after tax	707	257	(91)
Less: Net realized gains (losses) reclassified to earnings, after tax	324	28	46
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax	383	229	(137)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax [Abstract]
Net pension and other postretirement benefit plan activity and other reclassified to earnings, pre-tax	82	158	16
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax [Abstract]
Net pension and other postretirement benefit plan activity and other reclassified to earnings, tax	(19)	(36)	(4)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax [Abstract]
Net pension and other postretirement benefit plan activity and other reclassified to earnings, after-tax	63	122	12
Other Comprehensive Income Other Adjustments [Abstract]
Net unrealized gains (losses) on other transactions, pre-tax	10	17	18
Total Other, net activity, Before tax	10	17	18
Net unrealized gains (losses) on other transactions, tax effect	5	5	(17)
Effect of income taxes	5	5	(17)
Net unrealized gains (losses) on other transactions, after tax	15	22	1
Total Other, net activity, After tax	15	22	1
Other comprehensive income (loss), before tax and net of reclassifications into Net income	2,548	1,966	(739)
Income tax benefit (expense) related to items of other comprehensive income	(577)	(442)	156
Other comprehensive income (loss), after tax and net of reclassifications into Net income	1,971	1,524	(583)
Continuing Operations
Other Comprehensive Income Other Adjustments [Abstract]
Net unrealized gains (losses) on other transactions, pre-tax	10	17	18
Other comprehensive income (loss), before tax and net of reclassifications into Net income	2,400	1,961	(684)
Income tax benefit (expense) related to items of other comprehensive income	(544)	(441)	144
Other comprehensive income (loss), after tax and net of reclassifications into Net income	1,856	1,520	(540)
Discontinued Operations
Other Comprehensive Income Other Adjustments [Abstract]
Other comprehensive income (loss), before tax and net of reclassifications into Net income	148	5	(55)
Income tax benefit (expense) related to items of other comprehensive income	(33)	(1)	12
Other comprehensive income (loss), after tax and net of reclassifications into Net income	$ 115	$ 4	$ (43)