Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 8,307	$ 5,076
Net unrealized gains (losses)	(104)	102
Amounts reclassified from AOCI	3	2
Other comprehensive income (loss)	(101)	104
Balance, end of period	8,307	5,056
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	108	31
Net unrealized gains (losses)	(103)	89
Amounts reclassified from AOCI	0	0
Other comprehensive income (loss)	(103)	89
Balance, end of period	5	120
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	12	3
Net unrealized gains (losses)	(3)	13
Amounts reclassified from AOCI	1	0
Other comprehensive income (loss)	(2)	13
Balance, end of period	10	16
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(260)	(273)
Net unrealized gains (losses)	2	0
Amounts reclassified from AOCI	2	2
Other comprehensive income (loss)	4	2
Balance, end of period	(256)	(271)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(140)	(239)
Balance, end of period	$ (241)	$ (135)