Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Details) - USD ($)		Sep. 30, 2019	Dec. 31, 2018
ASSETS
Trading account assets		$ 963,000,000	$ 849,000,000
Loans held for sale			0
Derivative assets		1,143,000,000	531,000,000
Securities available for sale		22,378,000,000	19,428,000,000
Amortized Cost		10,490,000,000	11,519,000,000
Held-to-maturity securities		10,606,000,000	11,122,000,000
Loans, net of allowance		91,867,000,000	88,669,000,000
Loans held for sale	[1]	1,598,000,000	1,227,000,000
LIABILITIES
Long-term debt		14,470,000,000	13,732,000,000
Carrying Amount [Member]
ASSETS
Trading account assets		963,000,000	849,000,000
Other investments		620,000,000	666,000,000
Loans, net of allowance		3,000,000	3,000,000
Loans held for sale		101,000,000	54,000,000
Securities available for sale		22,378,000,000	19,428,000,000
Amortized Cost		10,490,000,000	11,519,000,000
Loans, net of allowance		91,864,000,000	88,666,000,000
Loans held for sale		1,497,000,000	1,173,000,000
Cash and short-term investments		3,987,000,000	3,240,000,000
LIABILITIES
Time deposits		12,494,000,000	13,245,000,000
Short-term borrowings		882,000,000	863,000,000
Long-term debt		14,470,000,000	13,732,000,000
Deposits with no stated maturity		99,155,000,000	94,064,000,000
Fair Value [Member]
ASSETS
Trading account assets		963,000,000	849,000,000
Other investments		620,000,000	666,000,000
Loans, net of allowance		3,000,000	3,000,000
Loans held for sale		101,000,000	54,000,000
Securities available for sale		22,378,000,000	19,428,000,000
Held-to-maturity securities			11,122,000,000
Loans, net of allowance		90,651,000,000	86,224,000,000
Loans held for sale		1,497,000,000	1,173,000,000
Cash and short-term investments		3,987,000,000	3,240,000,000
LIABILITIES
Time deposits		12,586,000,000	13,331,000,000
Short-term borrowings		882,000,000	863,000,000
Long-term debt		14,985,000,000	13,787,000,000
Deposits with no stated maturity		99,155,000,000	94,064,000,000
Fair Value [Member] | Level 1 [Member]
ASSETS
Trading account assets		0	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		3,987,000,000	3,240,000,000
LIABILITIES
Time deposits		0	0
Short-term borrowings		31,000,000	14,000,000
Long-term debt		13,915,000,000	12,576,000,000
Deposits with no stated maturity		0	0
Fair Value [Member] | Level 2 [Member]
ASSETS
Trading account assets		963,000,000	849,000,000
Other investments		0	1,000,000
Loans, net of allowance		0	0
Loans held for sale		100,000,000	54,000,000
Securities available for sale		22,367,000,000	19,408,000,000
Held-to-maturity securities		10,606,000,000	11,122,000,000
Loans, net of allowance		0	0
Loans held for sale		0	0
Cash and short-term investments		0	0
LIABILITIES
Time deposits		12,586,000,000	13,331,000,000
Short-term borrowings		851,000,000	849,000,000
Long-term debt		1,070,000,000	1,211,000,000
Deposits with no stated maturity		99,155,000,000	94,064,000,000
Fair Value [Member] | Level 3 [Member]
ASSETS
Trading account assets		0	0
Other investments		538,000,000	559,000,000
Loans, net of allowance		3,000,000	3,000,000
Loans held for sale		1,000,000	0
Securities available for sale		11,000,000	20,000,000
Held-to-maturity securities		0	0
Loans, net of allowance		90,651,000,000	86,224,000,000
Loans held for sale		1,497,000,000	1,173,000,000
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] | Measured at NAV [Member]
ASSETS
Other investments		82,000,000	106,000,000
Derivatives Designated as Hedging Instruments [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		116,000,000	69,000,000
LIABILITIES
Derivative liabilities		0	(9,000,000)
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member]
ASSETS
Derivative assets		116,000,000	69,000,000
Derivative assets, Netting Adjustment		(94,000,000)	17,000,000
LIABILITIES
Derivative liabilities		0	(9,000,000)
Derivative liabilities, Netting Adjustment		(28,000,000)	1,000,000
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 1 [Member]
ASSETS
Derivative assets		0	2,000,000
LIABILITIES
Derivative liabilities		0	0
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 2 [Member]
ASSETS
Derivative assets		210,000,000	50,000,000
LIABILITIES
Derivative liabilities		28,000,000	(10,000,000)
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		1,027,000,000	462,000,000
LIABILITIES
Derivative liabilities		322,000,000	395,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member]
ASSETS
Derivative assets		1,027,000,000	462,000,000
Derivative assets, Netting Adjustment		(398,000,000)	(350,000,000)
LIABILITIES
Derivative liabilities		322,000,000	395,000,000
Derivative liabilities, Netting Adjustment		(303,000,000)	(338,000,000)
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 1 [Member]
ASSETS
Derivative assets		60,000,000	68,000,000
LIABILITIES
Derivative liabilities		56,000,000	58,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 2 [Member]
ASSETS
Derivative assets		1,355,000,000	736,000,000
LIABILITIES
Derivative liabilities		563,000,000	675,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 3 [Member]
ASSETS
Derivative assets		10,000,000	8,000,000
LIABILITIES
Derivative liabilities		$ 6,000,000	$ 0

[1] Total loans held for sale include real estate — residential mortgage loans held for sale at fair value of $101 million at September 30, 2019, and $54 million at December 31, 2018.