Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	$ 733,000,000	$ 1,040,000,000
Securities available for sale	26,895,000,000	21,843,000,000
Loans, net of unearned income (residential)	103,081,000,000	94,646,000,000
Loans held for sale (residential)		0
Derivative assets, netting adjustments	(490,000,000)		$ (473,000,000)
Total derivative assets	1,946,000,000	780,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, netting adjustments	(679,000,000)		$ (335,000,000)
U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	4,464,000,000	334,000,000
States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	3,000,000	4,000,000
Other securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	12,000,000	11,000,000
Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,326,000,000	1,714,000,000
Mortgage-backed securities | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	7,358,000,000	6,997,000,000
Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	44,000,000
Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	1,000,000,000	0
Total trading account securities	716,000,000	995,000,000
Commercial loans	17,000,000	45,000,000
Total trading account assets	733,000,000	1,040,000,000
Securities available for sale	26,895,000,000	21,843,000,000
Total other investments	81,000,000	90,000,000
Loans, net of unearned income (residential)	8,000,000	4,000,000
Loans held for sale (residential)	288,000,000	140,000,000
Derivative assets, gross	2,436,000,000	1,253,000,000
Derivative assets, netting adjustments	(490,000,000)	(473,000,000)
Total derivative assets	1,946,000,000	780,000,000
Accrued income and other assets	0	0
Total assets at fair value	30,951,000,000	23,897,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	986,000,000	903,000,000
Recurring | U.S. Treasury Bills
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	1,000,000,000	0
Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	611,000,000	843,000,000
Securities available for sale	4,464,000,000	334,000,000
Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	52,000,000	30,000,000
Securities available for sale	3,000,000	4,000,000
Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	13,000,000	44,000,000
Securities available for sale	12,000,000	11,000,000
Recurring | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	7,358,000,000	6,997,000,000
Recurring | Principal investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1,000,000	1,000,000
Recurring | Principal Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	61,000,000	69,000,000
Recurring | Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	12,000,000	12,000,000
Recurring | Equity Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	20,000,000	21,000,000
Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	40,000,000	78,000,000
Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,326,000,000	1,714,000,000
Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	818,000,000	705,000,000
Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	847,000,000	533,000,000
Derivative liabilities, netting adjustments	(679,000,000)	(335,000,000)
Total derivative liabilities	168,000,000	198,000,000
Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,827,000,000	963,000,000
Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	306,000,000	253,000,000
Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	74,000,000	67,000,000
Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	68,000,000	60,000,000
Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	462,000,000	208,000,000
Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	445,000,000	200,000,000
Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	3,000,000	1,000,000
Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	15,000,000	10,000,000
Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	70,000,000	14,000,000
Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	13,000,000	10,000,000
Level 1 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	0	0
Total trading account securities	1,000,000	0
Commercial loans	0	0
Total trading account assets	1,000,000	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	56,000,000	49,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	56,000,000	49,000,000
Accrued income and other assets	0	0
Total assets at fair value	57,000,000	49,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	340,000,000	62,000,000
Level 1 | Recurring | U.S. Treasury Bills
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	0	0
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	1,000,000	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	289,000,000	19,000,000
Level 1 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	51,000,000	43,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	51,000,000	43,000,000
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	56,000,000	49,000,000
Level 1 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	51,000,000	43,000,000
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 short-term investments	1,000,000,000	0
Total trading account securities	688,000,000	995,000,000
Commercial loans	17,000,000	45,000,000
Total trading account assets	705,000,000	1,040,000,000
Securities available for sale	26,883,000,000	21,832,000,000
Total other investments	0	0
Loans, net of unearned income (residential)	0	0
Loans held for sale (residential)	287,000,000	140,000,000
Derivative assets, gross	2,253,000,000	1,176,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	2,253,000,000	1,176,000,000
Accrued income and other assets	0	0
Total assets at fair value	30,128,000,000	24,188,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	1,310,000,000	1,167,000,000
Level 2 | Recurring | U.S. Treasury Bills
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	1,000,000,000	0
Level 2 | Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	611,000,000	843,000,000
Securities available for sale	4,464,000,000	334,000,000
Level 2 | Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	52,000,000	30,000,000
Securities available for sale	3,000,000	4,000,000
Level 2 | Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	12,000,000	44,000,000
Securities available for sale	0	0
Level 2 | Recurring | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	7,358,000,000	6,997,000,000
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	13,000,000	78,000,000
Level 2 | Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,326,000,000	1,714,000,000
Level 2 | Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	529,000,000	686,000,000
Level 2 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	781,000,000	481,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	781,000,000	481,000,000
Level 2 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,746,000,000	941,000,000
Level 2 | Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	306,000,000	253,000,000
Level 2 | Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	18,000,000	18,000,000
Level 2 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	17,000,000	17,000,000
Level 2 | Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	462,000,000	208,000,000
Level 2 | Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	445,000,000	200,000,000
Level 2 | Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,000,000	0
Level 2 | Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	1,000,000
Level 2 | Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	26,000,000	9,000,000
Level 2 | Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	13,000,000	10,000,000
Level 3 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	0	0
Total trading account securities	27,000,000	0
Commercial loans	0	0
Total trading account assets	27,000,000	0
Securities available for sale	12,000,000	11,000,000
Total other investments	13,000,000	13,000,000
Loans, net of unearned income (residential)	8,000,000	4,000,000
Loans held for sale (residential)	1,000,000	0
Derivative assets, gross	127,000,000	28,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	127,000,000	28,000,000
Accrued income and other assets	0	0
Total assets at fair value	188,000,000	56,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	15,000,000	9,000,000
Level 3 | Recurring | U.S. Treasury Bills
ASSETS MEASURED ON A RECURRING BASIS
Total short-term investments	0	0
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	12,000,000	11,000,000
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	1,000,000	1,000,000
Level 3 | Recurring | Principal Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1,000,000	1,000,000
Level 3 | Recurring | Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	12,000,000	12,000,000
Level 3 | Recurring | Equity Investments
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	12,000,000	12,000,000
Level 3 | Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	27,000,000	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	15,000,000	9,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	15,000,000	9,000,000
Level 3 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	81,000,000	22,000,000
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	0
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	2,000,000	1,000,000
Level 3 | Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	15,000,000	9,000,000
Level 3 | Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross		5,000,000
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	1,000,000	1,000,000
Measured at NAV | Recurring | Equity Investments Indirect, NAV
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	7,000,000	8,000,000
Collateralized mortgage Obligations | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	13,732,000,000	12,783,000,000
Collateralized mortgage Obligations | Level 1 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Collateralized mortgage Obligations | Level 2 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	13,732,000,000	12,783,000,000
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	$ 60,000,000	$ 68,000,000