Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	$ 795,000,000	$ 1,040,000,000
Securities available for sale	20,807,000,000	21,843,000,000
Loans, net of unearned income of $575 and $603	103,198,000,000	94,646,000,000
Loans held for sale (residential)		0
Derivative assets, netting adjustments	(809,000,000)		$ (473,000,000)
Total derivative assets	2,068,000,000	780,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, netting adjustments	(595,000,000)		$ (335,000,000)
US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	341,000,000	334,000,000
US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	4,000,000	4,000,000
Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	9,000,000	11,000,000
Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,539,000,000	1,714,000,000
Other Mortgage-Backed Securities [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	6,869,000,000	6,997,000,000
Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	789,000,000	995,000,000
Commercial loans	6,000,000	45,000,000
Total trading account assets	795,000,000	1,040,000,000
Securities available for sale	20,807,000,000	21,843,000,000
Total other investments	84,000,000	90,000,000
Loans, net of unearned income of $575 and $603	3,000,000	4,000,000
Loans held for sale (residential)	152,000,000	140,000,000
Derivative assets, gross	2,877,000,000	1,253,000,000
Derivative assets, netting adjustments	(809,000,000)	(473,000,000)
Total derivative assets	2,068,000,000	780,000,000
Accrued income and other assets	0	0
Total assets at fair value	23,909,000,000	23,897,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	1,255,000,000	903,000,000
Recurring [Member] | US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	556,000,000	843,000,000
Securities available for sale	341,000,000	334,000,000
Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	19,000,000	30,000,000
Securities available for sale	4,000,000	4,000,000
Recurring [Member] | Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	38,000,000	44,000,000
Securities available for sale	9,000,000	11,000,000
Recurring [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	6,869,000,000	6,997,000,000
Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1,000,000	1,000,000
Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	65,000,000	69,000,000
Recurring [Member] | Equity Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	10,000,000	12,000,000
Recurring [Member] | Equity Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	19,000,000	21,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	176,000,000	78,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member] | Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,539,000,000	1,714,000,000
Recurring [Member] | Short [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	606,000,000	705,000,000
Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	1,244,000,000	533,000,000
Derivative liabilities, netting adjustments	(595,000,000)	(335,000,000)
Total derivative liabilities	649,000,000	198,000,000
Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,941,000,000	963,000,000
Recurring [Member] | Interest Rate [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	329,000,000	253,000,000
Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	164,000,000	67,000,000
Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	157,000,000	60,000,000
Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	704,000,000	208,000,000
Recurring [Member] | Commodity [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	690,000,000	200,000,000
Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	11,000,000	1,000,000
Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	28,000,000	10,000,000
Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	57,000,000	14,000,000
Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	40,000,000	10,000,000
Level 1 [Member] | Recurring [Member]
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 of $575 and $603	0	0
Loans held for sale (residential)	0	0
Derivative assets, gross	124,000,000	49,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	124,000,000	49,000,000
Accrued income and other assets	0	0
Total assets at fair value	124,000,000	49,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	241,000,000	62,000,000
Level 1 [Member] | Recurring [Member] | US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 [Member] | Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 [Member] | Recurring [Member] | Equity Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments		0
Level 1 [Member] | Recurring [Member] | Equity Investments Direct, NAV [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0
Level 1 [Member] | Recurring [Member] | Equity Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Level 1 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | Short [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	124,000,000	19,000,000
Level 1 [Member] | Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	117,000,000	43,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	117,000,000	43,000,000
Level 1 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 [Member] | Recurring [Member] | Interest Rate [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 1 [Member] | Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	124,000,000	49,000,000
Level 1 [Member] | Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	117,000,000	43,000,000
Level 1 [Member] | Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 [Member] | Recurring [Member] | Commodity [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 1 [Member] | Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 [Member] | Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 1 [Member] | Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 1 [Member] | Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 2 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	789,000,000	995,000,000
Commercial loans	6,000,000	45,000,000
Total trading account assets	795,000,000	1,040,000,000
Securities available for sale	20,798,000,000	21,832,000,000
Total other investments	0	0
Loans, net of unearned income of $575 and $603	0	0
Loans held for sale (residential)	142,000,000	140,000,000
Derivative assets, gross	2,629,000,000	1,176,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	2,629,000,000	1,176,000,000
Accrued income and other assets	0	0
Total assets at fair value	24,364,000,000	24,188,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	1,581,000,000	1,167,000,000
Level 2 [Member] | Recurring [Member] | US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	556,000,000	843,000,000
Securities available for sale	341,000,000	334,000,000
Level 2 [Member] | Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	19,000,000	30,000,000
Securities available for sale	4,000,000	4,000,000
Level 2 [Member] | Recurring [Member] | Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	38,000,000	44,000,000
Securities available for sale	0	0
Level 2 [Member] | Recurring [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	6,869,000,000	6,997,000,000
Level 2 [Member] | Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 [Member] | Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 [Member] | Recurring [Member] | Equity Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 [Member] | Recurring [Member] | Equity Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 2 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	176,000,000	78,000,000
Level 2 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,539,000,000	1,714,000,000
Level 2 [Member] | Recurring [Member] | Short [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	482,000,000	686,000,000
Level 2 [Member] | Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	1,099,000,000	481,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	1,099,000,000	481,000,000
Level 2 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,845,000,000	941,000,000
Level 2 [Member] | Recurring [Member] | Interest Rate [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	329,000,000	253,000,000
Level 2 [Member] | Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	40,000,000	18,000,000
Level 2 [Member] | Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	40,000,000	17,000,000
Level 2 [Member] | Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	704,000,000	208,000,000
Level 2 [Member] | Recurring [Member] | Commodity [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	690,000,000	200,000,000
Level 2 [Member] | Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	6,000,000	0
Level 2 [Member] | Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	1,000,000
Level 2 [Member] | Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	34,000,000	9,000,000
Level 2 [Member] | Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	40,000,000	10,000,000
Level 3 [Member] | Recurring [Member]
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	9,000,000	11,000,000
Total other investments	11,000,000	13,000,000
Loans, net of unearned income of $575 and $603	3,000,000	4,000,000
Loans held for sale (residential)	10,000,000	0
Derivative assets, gross	124,000,000	28,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	124,000,000	28,000,000
Accrued income and other assets	0	0
Total assets at fair value	157,000,000	56,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Total liabilities on a recurring basis at fair value	28,000,000	9,000,000
Level 3 [Member] | Recurring [Member] | US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 3 [Member] | Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 3 [Member] | Recurring [Member] | Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	9,000,000	11,000,000
Level 3 [Member] | Recurring [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 3 [Member] | Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments		1,000,000
Level 3 [Member] | Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1,000,000	1,000,000
Level 3 [Member] | Recurring [Member] | Equity Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments		12,000,000
Level 3 [Member] | Recurring [Member] | Equity Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	10,000,000	12,000,000
Level 3 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Level 3 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Level 3 [Member] | Recurring [Member] | Short [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	0	0
Level 3 [Member] | Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	28,000,000	9,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	28,000,000	9,000,000
Level 3 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross		22,000,000
Level 3 [Member] | Recurring [Member] | Interest Rate [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 [Member] | Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 3 [Member] | Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 [Member] | Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 3 [Member] | Recurring [Member] | Commodity [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 [Member] | Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	5,000,000	1,000,000
Level 3 [Member] | Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	28,000,000	9,000,000
Level 3 [Member] | Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross		5,000,000
Level 3 [Member] | Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Measured at NAV [Member] | Recurring [Member] | Principal Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	64,000,000	68,000,000
Measured at NAV [Member] | Recurring [Member] | Equity Investments Direct, NAV [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1,000,000	1,000,000
Measured at NAV [Member] | Recurring [Member] | Equity Investments Indirect, NAV [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	8,000,000	8,000,000
Collateralized Mortgage Obligations [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	12,045,000,000	12,783,000,000
Collateralized Mortgage Obligations [Member] | Level 1 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0	0
Collateralized Mortgage Obligations [Member] | Level 2 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	12,045,000,000	12,783,000,000
Collateralized Mortgage Obligations [Member] | Level 3 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	$ 0	$ 0