Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 8,565	$ 8,307	$ 8,307	$ 5,208	$ 5,056	$ 5,076	$ 8,307	$ 5,076
Net unrealized gains (losses)	(41)			(7)			(108)	95
Amounts reclassified from AOCI	3			1			7	4
Other comprehensive income (loss)	(38)	38	(101)	(6)	1	104	(101)	99
Balance, end of period	8,532	8,565	8,307	8,144	5,208	5,056	8,532	8,144
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	43		108	118		31	108	31
Net unrealized gains (losses)	(38)			(6)			(103)	81
Amounts reclassified from AOCI	0			1			0	1
Other comprehensive income (loss)	(38)			(5)			(103)	82
Balance, end of period	5	43		113	118		5	113
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	8		12	16		3	12	3
Net unrealized gains (losses)	0			(1)			(1)	14
Amounts reclassified from AOCI	(1)			(2)			(4)	(4)
Other comprehensive income (loss)	(1)			(3)			(5)	10
Balance, end of period	7	8		13	16		7	13
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(254)		(260)	(268)		(273)	(260)	(273)
Net unrealized gains (losses)	(3)			0			(4)	0
Amounts reclassified from AOCI	4			2			11	7
Other comprehensive income (loss)	1			2			7	7
Balance, end of period	(253)	(254)		(266)	(268)		(253)	(266)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(203)		$ (140)	(134)		$ (239)	(140)	(239)
Balance, end of period	$ (241)	$ (203)		$ (140)	$ (134)		$ (241)	$ (140)