Accumulated Other Comprehensive Income - Change in Each Component of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Change in accumulated other comprehensive loss balances:
Unrealized net gains (losses) on hedges arising during the period, before tax	$ 17	$ 54
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	5	14
Unrealized net gains (losses) on hedges arising during the period, net of tax	12	40	$ (35)	$ 352
Reclassification adjustment for net (gains) losses on hedges included in interest income for loans and interest expense on long-term debt, net of tax	(18)	(6)	(38)	8
Net change, before tax	(8)	46	(99)	488
Net change, tax	2	(12)	26	(128)
Net change, net of tax	(6)	34	(73)	360
Unrealized holding gains (losses) arising during the period on securities available for sale, net of tax	205	214	(275)	506
Reclassification adjustment for net (gains) losses on securities available for sale included in securities gains, net, net of tax	(25)	(42)	(25)	(81)
Amortization of net unrealized (gains) losses on held to maturity securities, net of tax	6	7	13	12
Net change, before tax	252	242	(388)	592
Net change, tax	(66)	(63)	101	(155)
Net change, net of tax	186	179	(287)	437
Amortization of prior service credit, net of tax	(5)	(5)	(10)	(10)
Recognized net actuarial (gain) loss, net of tax	23	20	46	41
Net change, before tax	25	21	49	42
Net change, tax	(7)	(6)	(13)	(11)
Net change, net of tax	18	15	36	31
Other, before tax	(3)		(3)
Other, tax	1		1
Other	(2)	0	(2)	0
Other Comprehensive Income (Loss), before Tax	266	309	(441)	1,122
Other Comprehensive Income (Loss), Tax	(70)	(81)	115	(294)
Other Comprehensive Income (Loss), Net of Tax	196	228	(326)	828
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	25	8
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	7	2
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax	277	290
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	72	76
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	34	57
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	9	15
Other Comprehensive Income Amortization of Net Unrealized Losses on Held To Maturity Securities Before Tax	(9)	(9)
Other Comprehensive Income Amortization of Net Unrealized Losses on Held To Maturity Securities Tax	(3)	(2)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	(6)	(7)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax	(1)	(2)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	(31)	(28)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax	$ (8)	$ (8)
Net Unrealized Gains (Losses) on Cash Flow Hedges
Change in accumulated other comprehensive loss balances:
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(47)	477
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			(52)	11
Reclassification from AOCI, Current Period, Tax			14	(3)
Other Comprehensive Income (Loss) before Reclassifications, Tax			12	(125)
Accumulated Net Investment Gain (Loss) Arising During the Period, Attributable to Parent
Change in accumulated other comprehensive loss balances:
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(372)	686
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			(34)	(110)
Reclassification from AOCI, Current Period, Tax			9	29
Other Comprehensive Income (Loss) before Reclassifications, Tax			97	(180)
Accumulated Net Investment Gain (Loss), Held to Maturity Securities, Attributable to Parent
Change in accumulated other comprehensive loss balances:
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			18	16
Reclassification from AOCI, Current Period, Tax			(5)	(4)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
Change in accumulated other comprehensive loss balances:
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(13)	(14)
Other Comprehensive Income (Loss) before Reclassifications, Tax			3	4
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
Change in accumulated other comprehensive loss balances:
Other Comprehensive Income (Loss), before Reclassifications, before Tax			62	56
Other Comprehensive Income (Loss) before Reclassifications, Tax			$ (16)	$ (15)