Accumulated Other Comprehensive Income - Activity in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), pre-tax activity	$ (156)	$ 595	$ 2,135	$ 2,272
Other comprehensive income (loss), tax effect	36	(138)	(496)	(525)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	22,335	20,671	21,203	16,250
Other comprehensive income, net of tax	(120)	457	1,639	1,747
Ending Balance	22,951	21,404	22,951	21,404
Net unrealized gains on available-for-sale debt securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, pre-tax activity	(42)	497	1,715	1,817
Other comprehensive income (loss), before reclassifications, tax effect	10	(118)	(409)	(430)
Other comprehensive income (loss), before reclassifications, net activity	(32)	379	1,306	1,387
Reclassification adjustment, pre-tax activity	(45)	(3)	(45)	(3)
Reclassification adjustment, tax effect	11	1	11	1
Reclassification adjustment, net activity	(34)	(2)	(34)	(2)
Other comprehensive income (loss), pre-tax activity	(87)	494	1,670	1,814
Other comprehensive income (loss), tax effect	21	(117)	(398)	(429)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	2,150	781	812	(227)
Other comprehensive income, net of tax	(66)	377	1,272	1,385
Ending Balance	2,084	1,158	2,084	1,158
Net unrealized gains on cash flow hedge derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, pre-tax activity	1	105	625	456
Other comprehensive income (loss), before reclassifications, tax effect	0	(22)	(131)	(95)
Other comprehensive income (loss), before reclassifications, net activity	1	83	494	361
Reclassification adjustment, pre-tax activity	(72)	(5)	(165)	(2)
Reclassification adjustment, tax effect	16	1	35	0
Reclassification adjustment, net activity	(56)	(4)	(130)	(2)
Other comprehensive income (loss), pre-tax activity	(71)	100	460	454
Other comprehensive income (loss), tax effect	16	(21)	(96)	(95)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	841	440	422	160
Other comprehensive income, net of tax	(55)	79	364	359
Ending Balance	786	519	786	519
Defined benefit pension plans, net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, pre-tax activity	(2)		(2)
Other comprehensive income (loss), before reclassifications, tax effect	0		0
Other comprehensive income (loss), before reclassifications, net activity	(2)		(2)
Reclassification adjustment, pre-tax activity	4	1	7	4
Reclassification adjustment, tax effect	(1)	0	(2)	(1)
Reclassification adjustment, net activity	3	1	5	3
Other comprehensive income (loss), pre-tax activity	2	1	5	4
Other comprehensive income (loss), tax effect	(1)	0	(2)	(1)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(40)	(43)	(42)	(45)
Other comprehensive income, net of tax	1	1	3	3
Ending Balance	(39)	(42)	(39)	(42)
AOCI
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	2,951	1,178	1,192	(112)
Ending Balance	$ 2,831	$ 1,635	$ 2,831	$ 1,635