Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	$ 645,000,000	$ 1,040,000,000
Securities available for sale	23,600,000,000	21,843,000,000
Loans, net of unearned income (residential)	106,159,000,000	94,646,000,000
Loans held for sale (residential)		0
Derivative assets, netting adjustments	(609,000,000)		$ (473,000,000)
Total derivative assets	2,093,000,000	780,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, netting adjustments	(629,000,000)		$ (335,000,000)
U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	3,465,000,000	334,000,000
States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	4,000,000	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	1,445,000,000	1,714,000,000
Equity Investments, Direct
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	12,000,000
Mortgage-backed securities | Agency commercial mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	6,727,000,000	6,997,000,000
Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	46,000,000
Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	636,000,000	995,000,000
Commercial loans	9,000,000	45,000,000
Total trading account assets	645,000,000	1,040,000,000
Securities available for sale	23,600,000,000	21,843,000,000
Total other investments	78,000,000	90,000,000
Loans, net of unearned income (residential)	5,000,000	4,000,000
Loans held for sale (residential)	250,000,000	140,000,000
Derivative assets, gross	2,702,000,000	1,253,000,000
Derivative assets, netting adjustments	(609,000,000)	(473,000,000)
Total derivative assets	2,093,000,000	780,000,000
Accrued income and other assets	0	0
Total assets at fair value	26,671,000,000	23,897,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	1,025,000,000	903,000,000
Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	531,000,000	843,000,000
Securities available for sale	3,465,000,000	334,000,000
Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	40,000,000	30,000,000
Securities available for sale	4,000,000	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	6,727,000,000	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	57,000,000	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	21,000,000	21,000,000
Recurring | Mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	52,000,000	78,000,000
Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,445,000,000	1,714,000,000
Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	715,000,000	705,000,000
Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	939,000,000	533,000,000
Derivative liabilities, netting adjustments	(629,000,000)	(335,000,000)
Total derivative liabilities	310,000,000	198,000,000
Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	2,020,000,000	963,000,000
Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	337,000,000	253,000,000
Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	108,000,000	67,000,000
Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	97,000,000	60,000,000
Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	487,000,000	208,000,000
Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	462,000,000	200,000,000
Recurring | Credit
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	4,000,000	1,000,000
Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	20,000,000	10,000,000
Recurring | Other
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	83,000,000	14,000,000
Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	23,000,000	10,000,000
Level 1 | 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	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	79,000,000	49,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	79,000,000	49,000,000
Accrued income and other assets	0	0
Total assets at fair value	79,000,000	49,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	354,000,000	62,000,000
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	0	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	285,000,000	19,000,000
Level 1 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	69,000,000	43,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	69,000,000	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	79,000,000	49,000,000
Level 1 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	69,000,000	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 trading account securities	600,000,000	995,000,000
Commercial loans	9,000,000	45,000,000
Total trading account assets	609,000,000	1,040,000,000
Securities available for sale	23,588,000,000	21,832,000,000
Total other investments	0	0
Loans, net of unearned income (residential)	0	0
Loans held for sale (residential)	250,000,000	140,000,000
Derivative assets, gross	2,522,000,000	1,176,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	2,522,000,000	1,176,000,000
Accrued income and other assets	0	0
Total assets at fair value	26,969,000,000	24,188,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	1,280,000,000	1,167,000,000
Level 2 | Recurring | U.S. Treasury, agencies and corporations
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	531,000,000	843,000,000
Securities available for sale	3,465,000,000	334,000,000
Level 2 | Recurring | States and political subdivisions
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	40,000,000	30,000,000
Securities available for sale	4,000,000	4,000,000
Level 2 | Recurring | Other securities
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	13,000,000	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	6,727,000,000	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	16,000,000	78,000,000
Level 2 | Recurring | Mortgage-backed securities | Agency residential mortgage-backed securities
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,445,000,000	1,714,000,000
Level 2 | Recurring | Short positions
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	430,000,000	686,000,000
Level 2 | Recurring | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	850,000,000	481,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	850,000,000	481,000,000
Level 2 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,968,000,000	941,000,000
Level 2 | Recurring | Interest rate | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	337,000,000	253,000,000
Level 2 | Recurring | Foreign exchange
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	29,000,000	18,000,000
Level 2 | Recurring | Foreign exchange | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	28,000,000	17,000,000
Level 2 | Recurring | Commodity
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	487,000,000	208,000,000
Level 2 | Recurring | Commodity | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	462,000,000	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	37,000,000	9,000,000
Level 2 | Recurring | Other | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	23,000,000	10,000,000
Level 3 | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	36,000,000	0
Commercial loans	0	0
Total trading account assets	36,000,000	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)	5,000,000	4,000,000
Loans held for sale (residential)	0	0
Derivative assets, gross	101,000,000	28,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	101,000,000	28,000,000
Accrued income and other assets	0	0
Total assets at fair value	167,000,000	56,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	20,000,000	9,000,000
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
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	36,000,000	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	20,000,000	9,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	20,000,000	9,000,000
Level 3 | Recurring | Interest rate
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	52,000,000	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	3,000,000	1,000,000
Level 3 | Recurring | Credit | Long positions
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	20,000,000	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	8,000,000	8,000,000
Collateralized mortgage Obligations | Recurring
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	11,947,000,000	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	11,947,000,000	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	$ 56,000,000	$ 68,000,000