Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Detail) - USD ($)		Sep. 30, 2018	Dec. 31, 2017
ASSETS
Trading account assets		$ 958,000,000	$ 836,000,000
Loans held for sale		0	0
Derivative assets		748,000,000	669,000,000
Securities available for sale		18,341,000,000	18,139,000,000
Amortized Cost		11,869,000,000	11,830,000,000
Held-to-maturity securities		11,261,000,000	11,565,000,000
Loans, net of allowance		88,381,000,000	85,528,000,000
Loans held for sale	[1]	1,618,000,000	1,107,000,000
LIABILITIES
Long-term debt		13,849,000,000	14,333,000,000
Carrying Amount [Member]
ASSETS
Trading account assets		958,000,000	836,000,000
Other investments		681,000,000	726,000,000
Loans, net of allowance		3,000,000	2,000,000
Loans held for sale		87,000,000	71,000,000
Securities available for sale		18,341,000,000	18,139,000,000
Amortized Cost		11,869,000,000	11,830,000,000
Loans, net of allowance		88,378,000,000	85,526,000,000
Loans held for sale		1,531,000,000	1,036,000,000
Cash and short-term investments		2,591,000,000	5,118,000,000
LIABILITIES
Time deposits		13,583,000,000	11,647,000,000
Short-term borrowings		1,922,000,000	1,011,000,000
Long-term debt		13,849,000,000	14,333,000,000
Deposits with no stated maturity		92,197,000,000	93,588,000,000
Fair Value [Member]
ASSETS
Trading account assets		958,000,000	836,000,000
Other investments		681,000,000	726,000,000
Loans, net of allowance		3,000,000	2,000,000
Loans held for sale		87,000,000	71,000,000
Securities available for sale		18,341,000,000	18,139,000,000
Held-to-maturity securities		11,261,000,000	11,565,000,000
Loans, net of allowance		86,418,000,000	84,003,000,000
Loans held for sale		1,531,000,000	1,036,000,000
Cash and short-term investments		2,591,000,000	5,118,000,000
LIABILITIES
Time deposits		13,654,000,000	11,750,000,000
Short-term borrowings		1,922,000,000	1,011,000,000
Long-term debt		13,864,000,000	14,626,000,000
Deposits with no stated maturity		92,197,000,000	93,588,000,000
Fair Value [Member] | Level 1 [Member]
ASSETS
Trading account assets		3,000,000	0
Other investments		0	0
Loans, net of allowance		0	0
Loans held for sale		0	0
Securities available for sale		0	0
Held-to-maturity securities		0	0
Loans, net of allowance		0	0
Loans held for sale		0	0
Cash and short-term investments		2,591,000,000	5,118,000,000
LIABILITIES
Time deposits		0	0
Short-term borrowings		37,000,000	72,000,000
Long-term debt		12,659,000,000	13,407,000,000
Deposits with no stated maturity		0	0
Fair Value [Member] | Level 2 [Member]
ASSETS
Trading account assets		955,000,000	836,000,000
Other investments		1,000,000	4,000,000
Loans, net of allowance		0	0
Loans held for sale		87,000,000	70,000,000
Securities available for sale		18,321,000,000	18,119,000,000
Held-to-maturity securities		11,261,000,000	11,565,000,000
Loans, net of allowance		0	0
Loans held for sale		0	0
Cash and short-term investments		0	0
LIABILITIES
Time deposits		13,654,000,000	11,750,000,000
Short-term borrowings		1,885,000,000	939,000,000
Long-term debt		1,205,000,000	1,219,000,000
Deposits with no stated maturity		92,197,000,000	93,588,000,000
Fair Value [Member] | Level 3 [Member]
ASSETS
Trading account assets		0	0
Other investments		578,000,000	598,000,000
Loans, net of allowance		3,000,000	2,000,000
Loans held for sale		0	1,000,000
Securities available for sale		20,000,000	20,000,000
Held-to-maturity securities		0	0
Loans, net of allowance		86,418,000,000	84,003,000,000
Loans held for sale		1,531,000,000	1,036,000,000
Cash and short-term investments		0	0
LIABILITIES
Time deposits		0	0
Short-term borrowings		0	0
Long-term debt		0	0
Deposits with no stated maturity		0	0
Fair Value [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
ASSETS
Other investments		102,000,000	124,000,000
Derivatives Designated as Hedging Instruments [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		48,000,000	(12,000,000)
LIABILITIES
Derivative liabilities		(3,000,000)	2,000,000
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member]
ASSETS
Derivative assets		48,000,000	(12,000,000)
Derivative assets, Netting Adjustment		8,000,000	(94,000,000)
LIABILITIES
Derivative liabilities		(3,000,000)	2,000,000
Derivative liabilities, Netting Adjustment			(48,000,000)
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 1 [Member]
ASSETS
Derivative assets		2,000,000	1,000,000
LIABILITIES
Derivative liabilities		1,000,000	4,000,000
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 2 [Member]
ASSETS
Derivative assets		38,000,000	81,000,000
LIABILITIES
Derivative liabilities		(4,000,000)	46,000,000
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 3 [Member]
ASSETS
Derivative assets		0	0
LIABILITIES
Derivative liabilities		0	0
Derivatives Not Designated as Hedging Instruments [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		700,000,000	681,000,000
LIABILITIES
Derivative liabilities		446,000,000	289,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member]
ASSETS
Derivative assets		700,000,000	681,000,000
Derivative assets, Netting Adjustment		(170,000,000)	(349,000,000)
LIABILITIES
Derivative liabilities		446,000,000	289,000,000
Derivative liabilities, Netting Adjustment		(582,000,000)	(568,000,000)
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 1 [Member]
ASSETS
Derivative assets		68,000,000	99,000,000
LIABILITIES
Derivative liabilities		61,000,000	94,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 2 [Member]
ASSETS
Derivative assets		794,000,000	918,000,000
LIABILITIES
Derivative liabilities		967,000,000	763,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 3 [Member]
ASSETS
Derivative assets		8,000,000	13,000,000
LIABILITIES
Derivative liabilities		$ 0	$ 0

[1] Total loans held for sale include real estate — residential mortgage loans held for sale at fair value of $87 million at September 30, 2018, and $71 million at December 31, 2017.