Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	$ 783,000,000	$ 867,000,000
Securities available for sale	19,012,000,000	20,212,000,000
Loans, net of unearned income	86,492,000,000	86,038,000,000	$ 85,528,000,000
Loans held for sale	0	0
Derivative Assets, Netting adjustments	(390,000,000)	(431,000,000)
Total derivative assets	622,000,000	803,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative Liabilities, Netting adjustments	(557,000,000)	(384,000,000)
Total derivative liabilities	232,000,000	636,000,000
Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	15,401,000,000	16,408,000,000
U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	158,000,000	184,000,000
States and Political Subdivisions [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	10,000,000	11,000,000
Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	20,000,000	20,000,000
Agency Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,541,000,000	1,846,000,000
Other Mortgage-Backed Securities [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,882,000,000	1,743,000,000
Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Commercial loans	7,000,000	18,000,000
Total trading account assets	783,000,000	867,000,000
Securities available for sale	19,012,000,000	20,212,000,000
Total other investments	153,000,000	191,000,000
Loans, net of unearned income	0	0
Loans held for sale	60,000,000	62,000,000
Derivative assets, gross	1,012,000,000	1,234,000,000
Derivative Assets, Netting adjustments	(390,000,000)	(431,000,000)
Total derivative assets	622,000,000	803,000,000
Accrued income and other assets	0	8,000,000
Total assets at fair value	20,630,000,000	22,143,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	789,000,000	1,020,000,000
Derivative Liabilities, Netting adjustments	(557,000,000)	(384,000,000)
Total derivative liabilities	232,000,000	636,000,000
Accrued expense and other liabilities	0	14,000,000
Total liabilities on a recurring basis at fair value	848,000,000	1,458,000,000
Recurring [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	15,401,000,000	16,408,000,000
Recurring [Member] | Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	776,000,000	849,000,000
Recurring [Member] | U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	582,000,000	655,000,000
Securities available for sale	158,000,000	184,000,000
Recurring [Member] | States and Political Subdivisions [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	33,000,000	8,000,000
Securities available for sale	10,000,000	11,000,000
Recurring [Member] | Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	37,000,000	73,000,000
Securities available for sale	20,000,000	20,000,000
Recurring [Member] | Agency Residential Mortgage Backed Securities [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		1,846,000,000
Recurring [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,882,000,000
Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	13,000,000	27,000,000
Recurring [Member] | Principal Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	138,000,000	158,000,000
Recurring [Member] | Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	151,000,000	185,000,000
Recurring [Member] | Equity And Mezzanine Indirect Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	2,000,000	6,000,000
Recurring [Member] | Private Equity and Mezzanine Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	2,000,000	6,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	124,000,000	113,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member] | Agency Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,541,000,000
Recurring [Member] | Other Mortgage-Backed Securities [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		1,743,000,000
Recurring [Member] | Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	616,000,000	808,000,000
Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	731,000,000	930,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	498,000,000	737,000,000
Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	129,000,000	123,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	137,000,000	113,000,000
Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	144,000,000	176,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	135,000,000	165,000,000
Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	1,000,000	1,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	6,000,000	4,000,000
Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	7,000,000	4,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	13,000,000	1,000,000
Level 1 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Commercial loans	0	0
Total trading account assets	2,000,000	0
Securities available for sale	0	3,000,000
Total other investments	0	0
Loans, net of unearned income	0	0
Loans held for sale	0	0
Derivative assets, gross	103,000,000	114,000,000
Derivative Assets, Netting adjustments	0	0
Total derivative assets	103,000,000	114,000,000
Accrued income and other assets	0	0
Total assets at fair value	105,000,000	117,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	114,000,000	102,000,000
Derivative Liabilities, Netting adjustments	0	0
Total derivative liabilities	114,000,000	102,000,000
Accrued expense and other liabilities	0	0
Total liabilities on a recurring basis at fair value	203,000,000	294,000,000
Level 1 [Member] | Recurring [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	2,000,000	0
Level 1 [Member] | Recurring [Member] | U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 1 [Member] | Recurring [Member] | States and Political Subdivisions [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	2,000,000	0
Securities available for sale	0	3,000,000
Level 1 [Member] | Recurring [Member] | Agency Residential Mortgage Backed Securities [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		0
Level 1 [Member] | Recurring [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	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 And Mezzanine Indirect Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 1 [Member] | Recurring [Member] | Private Equity and Mezzanine 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] | Agency Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0
Level 1 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		0
Level 1 [Member] | Recurring [Member] | Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	89,000,000	192,000,000
Level 1 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
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	103,000,000	114,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	114,000,000	102,000,000
Level 1 [Member] | Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
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
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
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 2 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Commercial loans	7,000,000	18,000,000
Total trading account assets	781,000,000	867,000,000
Securities available for sale	18,992,000,000	20,192,000,000
Total other investments	2,000,000	0
Loans, net of unearned income	0	0
Loans held for sale	58,000,000	62,000,000
Derivative assets, gross	893,000,000	1,110,000,000
Derivative Assets, Netting adjustments	0	0
Total derivative assets	893,000,000	1,110,000,000
Accrued income and other assets	0	8,000,000
Total assets at fair value	20,726,000,000	22,239,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	675,000,000	918,000,000
Derivative Liabilities, Netting adjustments	0	0
Total derivative liabilities	675,000,000	918,000,000
Accrued expense and other liabilities	0	14,000,000
Total liabilities on a recurring basis at fair value	1,202,000,000	1,548,000,000
Level 2 [Member] | Recurring [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	15,401,000,000	16,408,000,000
Level 2 [Member] | Recurring [Member] | Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	774,000,000	849,000,000
Level 2 [Member] | Recurring [Member] | U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	582,000,000	655,000,000
Securities available for sale	158,000,000	184,000,000
Level 2 [Member] | Recurring [Member] | States and Political Subdivisions [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	33,000,000	8,000,000
Securities available for sale	10,000,000	11,000,000
Level 2 [Member] | Recurring [Member] | Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	35,000,000	73,000,000
Securities available for sale	0	0
Level 2 [Member] | Recurring [Member] | Agency Residential Mortgage Backed Securities [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		1,846,000,000
Level 2 [Member] | Recurring [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,882,000,000
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 And Mezzanine Indirect Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	2,000,000	0
Level 2 [Member] | Recurring [Member] | Private Equity and Mezzanine Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	2,000,000	0
Level 2 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	124,000,000	113,000,000
Level 2 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Agency Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	1,541,000,000
Level 2 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		1,743,000,000
Level 2 [Member] | Recurring [Member] | Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	527,000,000	616,000,000
Level 2 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	720,000,000	923,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	498,000,000	737,000,000
Level 2 [Member] | Recurring [Member] | Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	26,000,000	9,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	23,000,000	11,000,000
Level 2 [Member] | Recurring [Member] | Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	144,000,000	176,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	135,000,000	165,000,000
Level 2 [Member] | Recurring [Member] | Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	0	0
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	6,000,000	4,000,000
Level 2 [Member] | Recurring [Member] | Other [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	3,000,000	2,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	13,000,000	1,000,000
Level 3 [Member] | Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Commercial loans	0	0
Total trading account assets	0	0
Securities available for sale	20,000,000	17,000,000
Total other investments	13,000,000	27,000,000
Loans, net of unearned income	0	0
Loans held for sale	2,000,000	0
Derivative assets, gross	16,000,000	10,000,000
Derivative Assets, Netting adjustments	0	0
Total derivative assets	16,000,000	10,000,000
Accrued income and other assets	0	0
Total assets at fair value	51,000,000	54,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Derivative Liabilities, Netting adjustments	0	0
Total derivative liabilities	0	0
Accrued expense and other liabilities	0	0
Total liabilities on a recurring basis at fair value	0	0
Level 3 [Member] | Recurring [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 3 [Member] | Recurring [Member] | Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Level 3 [Member] | Recurring [Member] | U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account securities	0	0
Securities available for sale	0	0
Level 3 [Member] | Recurring [Member] | States and Political Subdivisions [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	20,000,000	17,000,000
Level 3 [Member] | Recurring [Member] | Agency Residential Mortgage Backed Securities [Member] | Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		0
Level 3 [Member] | Recurring [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0
Level 3 [Member] | Recurring [Member] | Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	13,000,000	27,000,000
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	13,000,000	27,000,000
Level 3 [Member] | Recurring [Member] | Equity And Mezzanine Indirect Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
Level 3 [Member] | Recurring [Member] | Private Equity and Mezzanine Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	0	0
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] | Agency Residential Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	0
Level 3 [Member] | Recurring [Member] | Other Mortgage-Backed Securities [Member] | Agency Commercial Mortgage Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale		0
Level 3 [Member] | Recurring [Member] | Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	0	0
Level 3 [Member] | Recurring [Member] | Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets, gross	11,000,000	7,000,000
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
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
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	1,000,000	1,000,000
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	4,000,000	2,000,000
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities, gross	$ 0	$ 0