Investment Securities - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Investment securities held to maturity purchased	$ 3,299,000,000	$ 0		$ 29,000,000,000
Debt securities, held-to-maturity, allowance for credit loss	3,000,000
Pledged securities not separately reported	70,570,000,000	49,480,000,000
Investment securities available-for-sale	$ 59,048,000,000	53,815,000,000
Percentage of investment portfolio considered investment grade	99.00%
Provision (reversal) for credit loss	$ 88,000,000	10,000,000	$ 15,000,000
HTM securities writeoff	0
Losses from other-than-temporary impairment		0	3,000,000
Gross pre-tax unrealized losses	$ 123,000,000	$ 169,000,000
Number of securities in loss position | security	503	622
Gross losses from OTTI		$ 0	(3,000,000)
Held-to-Maturity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Provision (reversal) for credit loss	$ 3,000,000
Agency MBS
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Debt securities, available-for-sale, transferred security, at carrying value	8,600,000,000	3,980,000,000	2,130,000
Debt securities, available-for-sale, unrealized loss	120,000,000	49,000,000	(53,000,000)
ABS and municipal bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities sold	2,650,000,000	5,640,000,000	26,370,000,000
Pre-tax gain (loss) on sale of securities	4,000,000	1,000,000	9,000,000
Non-U.S. debt securities, mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Investment securities available-for-sale	$ 1,996,000,000	1,980,000,000
Accounting Standards Update 2017-12
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Investment securities available-for-sale		$ 1,220,000,000
Loss from sale of HTM securities			$ 36,000,000
Minimum | Agency MBS
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Debt securities, available-for-sale, securities transferred, Security Term	3 years
Maximum | Agency MBS
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Debt securities, available-for-sale, securities transferred, Security Term	37 years