Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	$ 902	$ 735
Securities available for sale	40,594	27,556
Loans, net of unearned income (residential)	98,609	101,185
Derivative assets, netting adjustments	(302)	(380)
Total derivative assets	2,553	1,798
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, netting adjustments	(1,911)	(675)
Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	6
Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	889	717
Commercial loans	13	18
Total trading account assets	902	735
Securities available for sale	40,594	27,556
Total other investments	80	81
Loans, net of unearned income (residential)	10	11
Loans held for sale (residential)	237	264
Derivative assets, gross	2,855	2,178
Derivative assets, netting adjustments	(302)	(380)
Total derivative assets	2,553	1,798
Accrued income and other assets	0	0
Total assets at fair value	44,376	30,445
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	1,027	910
Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	747	633
Securities available for sale	8,938	1,000
Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	91	24
Securities available for sale	0	0
Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	30	13
Securities available for sale	23	13
Recurring | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	10,121	10,106
Recurring | Principal investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1	1
Recurring | Principal Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	49	54
Recurring | Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	9	13
Recurring | Equity Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	31	27
Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	21	47
Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	5,408	2,164
Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	767	759
Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	2,171	826
Derivative liabilities, netting adjustments	(1,911)	(675)
Total derivative liabilities	260	151
Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	954	1,584
Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	273	288
Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	80	109
Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	75	103
Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,799	426
Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	1,805	408
Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	1
Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	7	11
Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	22	58
Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	11	16
Level 1 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	17	0
Commercial loans	0	0
Total trading account assets	17	0
Securities available for sale	0	0
Total other investments	0	0
Loans, net of unearned income (residential)	0	0
Loans held for sale (residential)	0	0
Derivative assets, gross	65	78
Derivative assets, netting adjustments	0	0
Total derivative assets	65	78
Accrued income and other assets	0	0
Total assets at fair value	82	78
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	272	328
Level 1 | Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 | Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 | Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	17	0
Securities available for sale	0	0
Level 1 | Recurring | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 1 | Recurring | Principal investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 | Recurring | Principal Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 | Recurring | Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 | Recurring | Equity Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 | Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Level 1 | Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 1 | Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	212	256
Level 1 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	60	72
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	60	72
Level 1 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 | Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 1 | Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	65	78
Level 1 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	60	72
Level 1 | Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 | Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 1 | Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 | Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 1 | Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 | Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 2 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	872	717
Commercial loans	13	18
Total trading account assets	885	735
Securities available for sale	40,572	27,543
Total other investments	0	0
Loans, net of unearned income (residential)	0	0
Loans held for sale (residential)	237	264
Derivative assets, gross	2,750	2,009
Derivative assets, netting adjustments	0	0
Total derivative assets	2,750	2,009
Accrued income and other assets	0	0
Total assets at fair value	44,444	30,551
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	2,660	1,246
Level 2 | Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	747	633
Securities available for sale	8,938	1,000
Level 2 | Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	91	24
Securities available for sale	0	0
Level 2 | Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	13	13
Securities available for sale	1	0
Level 2 | Recurring | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	10,121	10,106
Level 2 | Recurring | Principal investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 | Recurring | Principal Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 | Recurring | Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 | Recurring | Equity Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 | Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	21	47
Level 2 | Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	5,408	2,164
Level 2 | Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	555	503
Level 2 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	2,105	743
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	2,105	743
Level 2 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	920	1,528
Level 2 | Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	273	288
Level 2 | Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	15	31
Level 2 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	15	31
Level 2 | Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,799	424
Level 2 | Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	1,805	408
Level 2 | Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 2 | Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	1	0
Level 2 | Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	16	26
Level 2 | Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	11	16
Level 3 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Commercial loans	0	0
Total trading account assets	0	0
Securities available for sale	22	13
Total other investments	10	14
Loans, net of unearned income (residential)	10	11
Loans held for sale (residential)	0	0
Derivative assets, gross	40	91
Derivative assets, netting adjustments	0	0
Total derivative assets	40	91
Accrued income and other assets	0	0
Total assets at fair value	82	129
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	6	11
Level 3 | Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 3 | Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 3 | Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	22	13
Level 3 | Recurring | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 3 | Recurring | Principal investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1	1
Level 3 | Recurring | Principal Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1	1
Level 3 | Recurring | Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	9	13
Level 3 | Recurring | Equity Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	9	13
Level 3 | Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Level 3 | Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 3 | Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	0	0
Level 3 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	6	11
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	6	11
Level 3 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	34	56
Level 3 | Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 | Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 3 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 | Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	2
Level 3 | Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 | Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	1
Level 3 | Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	6	11
Level 3 | Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross		32
Level 3 | Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Measured at NAV | Recurring | Equity Investments Direct, NAV
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	16	7
Measured at NAV | Recurring | Equity Investments Indirect, NAV
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	6	7
Agency residential collateralized mortgage obligations | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	16,104	14,273
Agency residential collateralized mortgage obligations | Level 1 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Agency residential collateralized mortgage obligations | Level 2 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	16,104	14,273
Agency residential collateralized mortgage obligations | Level 3 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Variable Interest Entity, Not Primary Beneficiary | Measured at NAV | Recurring | Principal Investments, Indirect
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	$ 48	$ 53