Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	$ 958,000,000	$ 836,000,000
Securities available for sale	18,341,000,000	18,139,000,000
Loans, net of unearned income	89,268,000,000	86,405,000,000
Loans held for sale	0	0
Derivative assets, netting adjustments	(162,000,000)	(443,000,000)
Total derivative assets	748,000,000	669,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, netting adjustments	(582,000,000)	(616,000,000)
US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	145,000,000	157,000,000
US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	7,000,000	9,000,000
Other Debt Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	20,000,000	20,000,000
Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,661,000,000	1,439,000,000
Other Mortgage-Backed Securities [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	2,316,000,000	1,854,000,000
Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	941,000,000	821,000,000
Commercial loans	17,000,000	15,000,000
Total trading account assets	958,000,000	836,000,000
Securities available for sale	18,341,000,000	18,139,000,000
Total other investments	122,000,000	144,000,000
Loans, net of unearned income	3,000,000	2,000,000
Loans held for sale	87,000,000	71,000,000
Derivative assets, gross	910,000,000	1,112,000,000
Derivative assets, netting adjustments	(162,000,000)	(443,000,000)
Total derivative assets	748,000,000	669,000,000
Accrued income and other assets	0	0
Total assets at fair value	20,259,000,000	19,861,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Accrued expense and other liabilities	0	0
Total liabilities on a recurring basis at fair value	1,080,000,000	925,000,000
Recurring [Member] | US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	759,000,000	615,000,000
Securities available for sale	145,000,000	157,000,000
Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	54,000,000	37,000,000
Securities available for sale	7,000,000	9,000,000
Recurring [Member] | Other Debt Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	42,000,000	65,000,000
Securities available for sale	20,000,000	20,000,000
Recurring [Member] | Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	2,316,000,000	1,854,000,000
Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	12,000,000	13,000,000
Recurring [Member] | Principal Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	101,000,000	124,000,000
Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	113,000,000	137,000,000
Recurring [Member] | Equity Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	8,000,000	7,000,000
Recurring [Member] | Equity Investments Direct, NAV [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	1,000,000	0
Recurring [Member] | Equity Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	9,000,000	7,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	86,000,000	104,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member] | Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,661,000,000	1,439,000,000
Recurring [Member] | Short [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	637,000,000	634,000,000
Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	1,025,000,000	907,000,000
Derivative liabilities, netting adjustments	(582,000,000)	(616,000,000)
Total derivative liabilities	443,000,000	291,000,000
Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	310,000,000	722,000,000
Recurring [Member] | Interest Rate [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	445,000,000	520,000,000
Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	102,000,000	130,000,000
Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	93,000,000	124,000,000
Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	491,000,000	255,000,000
Recurring [Member] | Commodity [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	481,000,000	246,000,000
Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	1,000,000
Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	3,000,000	4,000,000
Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	7,000,000	4,000,000
Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	3,000,000	13,000,000
Level 1 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	3,000,000	0
Commercial loans	0	0
Total trading account assets	3,000,000	0
Securities available for sale	0	0
Total other investments	0	0
Loans, net of unearned income	0	0
Loans held for sale	0	0
Derivative assets, gross	70,000,000	100,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	70,000,000	100,000,000
Accrued income and other assets	0	0
Total assets at fair value	73,000,000	100,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Accrued expense and other liabilities	0	0
Total liabilities on a recurring basis at fair value	99,000,000	170,000,000
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 Debt Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	3,000,000	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 Indirect [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	0
Level 1 [Member] | Recurring [Member] | Equity Investments Direct, NAV [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	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	37,000,000	72,000,000
Level 1 [Member] | Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	62,000,000	98,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	62,000,000	98,000,000
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	70,000,000	100,000,000
Level 1 [Member] | Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	62,000,000	98,000,000
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	938,000,000	821,000,000
Commercial loans	17,000,000	15,000,000
Total trading account assets	955,000,000	836,000,000
Securities available for sale	18,321,000,000	18,119,000,000
Total other investments	1,000,000	4,000,000
Loans, net of unearned income	0	0
Loans held for sale	87,000,000	70,000,000
Derivative assets, gross	832,000,000	999,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	832,000,000	999,000,000
Accrued income and other assets	0	0
Total assets at fair value	20,196,000,000	20,028,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Accrued expense and other liabilities	0	0
Total liabilities on a recurring basis at fair value	1,563,000,000	1,371,000,000
Level 2 [Member] | Recurring [Member] | US Treasury and Government [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	759,000,000	615,000,000
Securities available for sale	145,000,000	157,000,000
Level 2 [Member] | Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	54,000,000	37,000,000
Securities available for sale	7,000,000	9,000,000
Level 2 [Member] | Recurring [Member] | Other Debt Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	39,000,000	65,000,000
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	2,316,000,000	1,854,000,000
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 Indirect [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	1,000,000	4,000,000
Level 2 [Member] | Recurring [Member] | Equity Investments Direct, NAV [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	1,000,000	4,000,000
Level 2 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	86,000,000	104,000,000
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	1,661,000,000	1,439,000,000
Level 2 [Member] | Recurring [Member] | Short [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Short positions	600,000,000	562,000,000
Level 2 [Member] | Recurring [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	963,000,000	809,000,000
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	963,000,000	809,000,000
Level 2 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	304,000,000	713,000,000
Level 2 [Member] | Recurring [Member] | Interest Rate [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	445,000,000	520,000,000
Level 2 [Member] | Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	32,000,000	30,000,000
Level 2 [Member] | Recurring [Member] | Foreign Exchange [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	31,000,000	26,000,000
Level 2 [Member] | Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	491,000,000	255,000,000
Level 2 [Member] | Recurring [Member] | Commodity [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	481,000,000	246,000,000
Level 2 [Member] | Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
Level 2 [Member] | Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	3,000,000	4,000,000
Level 2 [Member] | Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	5,000,000	1,000,000
Level 2 [Member] | Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	3,000,000	13,000,000
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	20,000,000	20,000,000
Total other investments	19,000,000	16,000,000
Loans, net of unearned income	3,000,000	2,000,000
Loans held for sale	0	1,000,000
Derivative assets, gross	8,000,000	13,000,000
Derivative assets, netting adjustments	0	0
Total derivative assets	8,000,000	13,000,000
Accrued income and other assets	0	0
Total assets at fair value	50,000,000	52,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Accrued expense and other liabilities	0	0
Total liabilities on a recurring basis at fair value	0	0
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 Debt Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	20,000,000	20,000,000
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	12,000,000	13,000,000
Level 3 [Member] | Recurring [Member] | Principal Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 3 [Member] | Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	12,000,000	13,000,000
Level 3 [Member] | Recurring [Member] | Equity Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	7,000,000	3,000,000
Level 3 [Member] | Recurring [Member] | Equity Investments Direct, NAV [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 3 [Member] | Recurring [Member] | Equity Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	7,000,000	3,000,000
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	0	0
Derivative liabilities, netting adjustments	0	0
Total derivative liabilities	0	0
Level 3 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	6,000,000	9,000,000
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	0	1,000,000
Level 3 [Member] | Recurring [Member] | Credit [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 [Member] | Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	2,000,000	3,000,000
Level 3 [Member] | Recurring [Member] | Other [Member] | Long [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Collateralized Mortgage Obligations [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	14,192,000,000	14,660,000,000
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	14,192,000,000	14,660,000,000
Collateralized Mortgage Obligations [Member] | Level 3 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	$ 0	$ 0