Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Details) - USD ($)		Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
ASSETS
Trading account assets		$ 795,000,000	$ 1,040,000,000
Loans held for sale			0
Derivative assets		2,068,000,000	780,000,000
Securities available for sale		20,807,000,000	21,843,000,000
Amortized Cost		9,638,000,000	10,067,000,000
Held-to-maturity securities		10,012,000,000	10,116,000,000
Loans, net of allowance		103,198,000,000	94,646,000,000
Loans held for sale	[1]	2,143,000,000	1,334,000,000
LIABILITIES
Long-term debt		13,732,000,000	$ 12,448,000,000
Carrying Amount [Member]
ASSETS
Trading account assets		795,000,000		$ 1,040,000,000
Other investments		679,000,000		605,000,000
Loans, net of allowance		3,000,000		4,000,000
Loans held for sale		152,000,000		140,000,000
Securities available for sale		20,807,000,000		21,843,000,000
Amortized Cost		9,638,000,000		10,067,000,000
Loans, net of allowance		101,836,000,000		93,742,000,000
Loans held for sale		1,991,000,000		1,194,000,000
Cash and short-term investments		4,938,000,000		2,004,000,000
LIABILITIES
Time deposits		10,253,000,000		11,652,000,000
Short-term borrowings		7,050,000,000		1,092,000,000
Long-term debt		13,732,000,000		12,448,000,000
Deposits with no stated maturity		105,051,000,000		100,218,000,000
Fair Value [Member]
ASSETS
Trading account assets		795,000,000		1,040,000,000
Other investments		679,000,000		605,000,000
Loans, net of allowance		3,000,000		4,000,000
Loans held for sale		152,000,000		140,000,000
Securities available for sale		20,807,000,000		21,843,000,000
Held-to-maturity securities		10,012,000,000		10,116,000,000
Loans, net of allowance		99,535,000,000		92,641,000,000
Loans held for sale		1,991,000,000		1,194,000,000
Cash and short-term investments		4,938,000,000		2,004,000,000
LIABILITIES
Time deposits		10,336,000,000		11,752,000,000
Short-term borrowings		7,050,000,000		1,092,000,000
Long-term debt		13,859,000,000		12,943,000,000
Deposits with no stated maturity		105,051,000,000		100,218,000,000
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		4,938,000,000		2,004,000,000
LIABILITIES
Time deposits		0		0
Short-term borrowings		124,000,000		19,000,000
Long-term debt		13,144,000,000		12,694,000,000
Deposits with no stated maturity		0		0
Fair Value [Member] | Level 2 [Member]
ASSETS
Trading account assets		795,000,000		1,040,000,000
Other investments		0		0
Loans, net of allowance		0		0
Loans held for sale		142,000,000		140,000,000
Securities available for sale		20,798,000,000		21,832,000,000
Held-to-maturity securities		10,012,000,000		10,116,000,000
Loans, net of allowance		0		0
Loans held for sale		0		0
Cash and short-term investments		0		0
LIABILITIES
Time deposits		10,336,000,000		11,752,000,000
Short-term borrowings		6,926,000,000		1,073,000,000
Long-term debt		715,000,000		249,000,000
Deposits with no stated maturity		105,051,000,000		100,218,000,000
Fair Value [Member] | Level 3 [Member]
ASSETS
Trading account assets		0		0
Other investments		606,000,000		528,000,000
Loans, net of allowance		3,000,000		4,000,000
Loans held for sale		10,000,000		0
Securities available for sale		9,000,000		11,000,000
Held-to-maturity securities		0		0
Loans, net of allowance		99,535,000,000		92,641,000,000
Loans held for sale		1,991,000,000		1,194,000,000
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		73,000,000		77,000,000
Derivatives Designated as Hedging Instruments [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		60,000,000		65,000,000
LIABILITIES
Derivative liabilities		0		4,000,000
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member]
ASSETS
Derivative assets		60,000,000		65,000,000
Derivative assets, Netting Adjustment		(6,000,000)		(126,000,000)
LIABILITIES
Derivative liabilities		0		4,000,000
Derivative liabilities, Netting Adjustment		(3,000,000)		(16,000,000)
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 1 [Member]
ASSETS
Derivative assets		0		0
LIABILITIES
Derivative liabilities		0		0
Derivatives Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 2 [Member]
ASSETS
Derivative assets		66,000,000		191,000,000
LIABILITIES
Derivative liabilities		3,000,000		20,000,000
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		2,008,000,000		715,000,000
LIABILITIES
Derivative liabilities		649,000,000		194,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member]
ASSETS
Derivative assets		2,008,000,000		715,000,000
Derivative assets, Netting Adjustment		(803,000,000)		(347,000,000)
LIABILITIES
Derivative liabilities		649,000,000		194,000,000
Derivative liabilities, Netting Adjustment		(592,000,000)		(319,000,000)
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 1 [Member]
ASSETS
Derivative assets		124,000,000		49,000,000
LIABILITIES
Derivative liabilities		117,000,000		43,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 2 [Member]
ASSETS
Derivative assets		2,563,000,000		985,000,000
LIABILITIES
Derivative liabilities		1,096,000,000		461,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value [Member] | Level 3 [Member]
ASSETS
Derivative assets		124,000,000		28,000,000
LIABILITIES
Derivative liabilities		$ 28,000,000		$ 9,000,000

[1] Total loans held for sale include real estate — residential mortgage loans held for sale at fair value of $152 million at March 31, 2020, and $140 million at December 31, 2019.