Stockholders' Equity and Earnings per Share - Changes in AOCI by Component, Net of Tax (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 9,122	$ 4,477
Total other comprehensive income (loss), net of taxes and reclassification adjustments	449	258
Ending balance	9,504	4,665
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	438	(135)
Other comprehensive income (loss) before reclassifications	448	271
Amounts reclassified from accumulated other comprehensive income	1	(13)
Total other comprehensive income (loss), net of taxes and reclassification adjustments	449	258
Ending balance	887	123
AOCI related to available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	618	83
Other comprehensive income (loss) before reclassifications	446	273
Amounts reclassified from accumulated other comprehensive income	(8)	(27)
Total other comprehensive income (loss), net of taxes and reclassification adjustments	438	246
Ending balance	1,056	329
AOCI related to cash flow hedge relationships
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(244)	(315)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	13	18
Total other comprehensive income (loss), net of taxes and reclassification adjustments	13	18
Ending balance	(231)	(297)
AOCI related to defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	64	97
Other comprehensive income (loss) before reclassifications	2	(2)
Amounts reclassified from accumulated other comprehensive income	(4)	(4)
Total other comprehensive income (loss), net of taxes and reclassification adjustments	(2)	(6)
Ending balance	$ 62	$ 91