Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 8,307	$ 8,307	$ 5,056	$ 5,076	$ 8,307	$ 5,076
Net unrealized gains (losses)	34		1		(71)	102
Amounts reclassified from AOCI	4		0		8	3
Other comprehensive income (loss)	38	(101)	1	104	(63)	105
Balance, end of period	8,565	8,307	5,208	5,056	8,565	5,208
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	5	108	119	31	108	31
Net unrealized gains (losses)	38		(1)		(65)	87
Amounts reclassified from AOCI	0		0		0	0
Other comprehensive income (loss)	38		(1)		(65)	87
Balance, end of period	43	5	118	119	43	118
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	10	12	16	3	12	3
Net unrealized gains (losses)	0		2		(1)	15
Amounts reclassified from AOCI	(2)		(2)		(3)	(2)
Other comprehensive income (loss)	(2)		0		(4)	13
Balance, end of period	8	10	16	16	8	16
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(256)	(260)	(271)	(274)	(260)	(274)
Net unrealized gains (losses)	(4)		0		(5)	0
Amounts reclassified from AOCI	6		2		11	5
Other comprehensive income (loss)	2		2		6	5
Balance, end of period	(254)	(256)	(269)	(271)	(254)	(269)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(241)	(140)	(136)	(240)	(140)	(240)
Balance, end of period	$ (203)	$ (241)	$ (135)	$ (136)	$ (203)	$ (135)