Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Detail) - USD ($)	Sep. 30, 2017		Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016		Sep. 30, 2016
ASSETS
Trading account assets	$ 783,000,000				$ 867,000,000
Securities available for sale	19,012,000,000				20,212,000,000
Held-to-maturity securities	10,276,000,000				10,232,000,000
Held-to-maturity securities	10,109,000,000				10,007,000,000
Other investments	728,000,000				738,000,000
Loans, net of allowance	85,612,000,000				85,180,000,000
Loans, net of allowance	2,000,000
Loans held for sale	1,341,000,000	[1]	$ 1,743,000,000	$ 1,384,000,000	1,104,000,000	[1]	$ 1,137,000,000
Loans held for sale	0				0
Derivative assets	622,000,000				803,000,000
LIABILITIES
Long-term debt	15,100,000,000				12,384,000,000
Derivative liabilities	232,000,000				636,000,000
Carrying Amount [Member]
ASSETS
Cash and short-term investments	4,555,000,000				3,452,000,000
Trading account assets	783,000,000				867,000,000
Securities available for sale	19,012,000,000				20,212,000,000
Held-to-maturity securities	10,276,000,000				10,232,000,000
Other investments	728,000,000				738,000,000
Loans, net of allowance	85,612,000,000				85,180,000,000
Loans held for sale	1,341,000,000				1,104,000,000
Derivative assets	622,000,000				803,000,000
LIABILITIES
Deposits with no stated maturity	92,207,000,000				93,906,000,000
Time deposits	11,239,000,000				10,181,000,000
Short-term borrowings	988,000,000				2,310,000,000
Long-term debt	15,100,000,000				12,384,000,000
Derivative liabilities	232,000,000				636,000,000
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on non recurring basis	0				0
ASSETS
Cash and short-term investments	4,555,000,000				3,452,000,000
Trading account assets	783,000,000				867,000,000
Securities available for sale	19,012,000,000				20,212,000,000
Held-to-maturity securities	10,109,000,000				10,007,000,000
Other investments	724,000,000				733,000,000
Loans, net of allowance	84,211,000,000				83,285,000,000
Loans held for sale	1,341,000,000				1,104,000,000
Derivative assets	622,000,000				803,000,000
Derivative assets, Netting Adjustment	(390,000,000)				(431,000,000)
LIABILITIES
Deposits with no stated maturity	92,207,000,000				93,906,000,000
Time deposits	11,342,000,000				10,267,000,000
Short-term borrowings	988,000,000				2,310,000,000
Long-term debt	15,493,000,000				12,690,000,000
Derivative liabilities	232,000,000				636,000,000
Derivative liabilities, Netting Adjustment	(557,000,000)				(384,000,000)
Fair Value [Member] | Level 1 [Member]
ASSETS
Cash and short-term investments	4,555,000,000				3,452,000,000
Trading account assets	2,000,000				0
Securities available for sale	0				3,000,000
Held-to-maturity securities	0				0
Other investments	0				0
Loans, net of allowance	0				0
Loans held for sale	0				0
Derivative assets	103,000,000				114,000,000
LIABILITIES
Deposits with no stated maturity	0				0
Time deposits	0				0
Short-term borrowings	89,000,000				192,000,000
Long-term debt	13,752,000,000				12,386,000,000
Derivative liabilities	114,000,000				102,000,000
Fair Value [Member] | Level 2 [Member]
ASSETS
Cash and short-term investments	0				0
Trading account assets	781,000,000				867,000,000
Securities available for sale	18,992,000,000				20,192,000,000
Held-to-maturity securities					10,007,000,000
Other investments	2,000,000				0
Loans, net of allowance	0				0
Loans held for sale	58,000,000				62,000,000
Derivative assets	893,000,000				1,110,000,000
LIABILITIES
Deposits with no stated maturity	92,207,000,000				93,906,000,000
Time deposits	11,342,000,000				10,267,000,000
Short-term borrowings	899,000,000				2,118,000,000
Long-term debt	1,741,000,000				304,000,000
Derivative liabilities	675,000,000				918,000,000
Fair Value [Member] | Level 3 [Member]
ASSETS
Cash and short-term investments	0				0
Trading account assets	0				0
Securities available for sale	20,000,000				17,000,000
Held-to-maturity securities	0				0
Other investments	584,000,000				569,000,000
Loans, net of allowance	84,211,000,000				83,285,000,000
Loans held for sale	1,283,000,000				1,042,000,000
Derivative assets	16,000,000				10,000,000
LIABILITIES
Deposits with no stated maturity	0				0
Time deposits	0				0
Short-term borrowings	0				0
Long-term debt	0				0
Derivative liabilities	0				0
Other Investments [Member] | Fair Value [Member]
ASSETS
Other investments, Measured at NAV	$ 138,000,000				$ 164,000,000

[1] Total loans held for sale include real estate — residential mortgage loans held for sale at fair value of $60 million at September 30, 2017, and $62 million at December 31, 2016.