Investment Securities (Summary) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	$ 122,409	$ 84,172
Securities available for sale debt securities, unrealized gains	2,066	3,302
Securities available for sale debt securities, unrealized losses	(348)	(116)
Investment securities – available for sale	124,127	87,358
Held to maturity securities, amortized cost	1,479	1,441
Held-to-maturity securities, unrealized gains	111	167
Held-to-maturity securities, unrealized losses	(7)	(3)
Held-to-maturity securities, fair value	1,583	1,605
Debt securities, available-for-sale, accrued interest	284	238
Debt securities, available-for-sale, allowance for credit loss	$ 130	$ 79
Credit Concentration Risk \| Fitch, AAA or AA Rating \| Held-to-maturity Securities
Debt Securities, Held-to-maturity [Abstract]
Concentration risk, percentage	84.00%	85.00%
U.S. Treasury and government agencies
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	$ 40,628	$ 19,821
Securities available for sale debt securities, unrealized gains	472	903
Securities available for sale debt securities, unrealized losses	(131)	(13)
Investment securities – available for sale	40,969	20,711
Held to maturity securities, amortized cost	809	795
Held-to-maturity securities, unrealized gains	82	125
Held-to-maturity securities, fair value	891	920
Residential mortgage-backed \| Mortgage-backed Securities Agency
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	64,412	47,355
Securities available for sale debt securities, unrealized gains	957	1,566
Securities available for sale debt securities, unrealized losses	(184)	(10)
Investment securities – available for sale	65,185	48,911
Residential mortgage-backed \| Mortgage-backed Securities Non-agency
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	1,003	1,272
Securities available for sale debt securities, unrealized gains	242	243
Securities available for sale debt securities, unrealized losses	(3)	(14)
Investment securities – available for sale	1,242	1,501
Commercial mortgage-backed \| Mortgage-backed Securities Agency
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	2,008	2,571
Securities available for sale debt securities, unrealized gains	56	119
Securities available for sale debt securities, unrealized losses	(4)	(2)
Investment securities – available for sale	2,060	2,688
Commercial mortgage-backed \| Mortgage-backed Securities Non-agency
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	3,637	3,678
Securities available for sale debt securities, unrealized gains	48	78
Securities available for sale debt securities, unrealized losses	(14)	(67)
Investment securities – available for sale	3,671	3,689
Asset-backed
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	5,999	5,060
Securities available for sale debt securities, unrealized gains	75	100
Securities available for sale debt securities, unrealized losses	(7)	(10)
Investment securities – available for sale	6,067	5,150
Other
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Securities available for sale debt securities, amortized cost	4,722	4,415
Securities available for sale debt securities, unrealized gains	216	293
Securities available for sale debt securities, unrealized losses	(5)
Investment securities – available for sale	4,933	4,708
Held to maturity securities, amortized cost	670	646
Held-to-maturity securities, unrealized gains	29	42
Held-to-maturity securities, unrealized losses	(7)	(3)
Held-to-maturity securities, fair value	$ 692	$ 685

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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