Fair Value Measurement and Fair Value of Financial Instruments - Reconciliation of Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2017	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, end of period	$ 1			$ 1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Settlements		$ 0
Trading derivatives
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(2)	(12)			$ (13)
Included in income before taxes	1	0			2
Included in other comprehensive income	0	0			0
Purchases/additions	0	0			0
Settlements	2	0			3
Asset (liability) balance, end of period	(1)	(12)		(1)	(12)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(1)	0			(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net				0	0
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(108)	(125)		(124)	(118)
Included in income before taxes	5	0		3	7
Included in other comprehensive income	0	0		0	0
Purchases/additions	0	0		0	0
Settlements	108			122	0
Asset (liability) balance, end of period	(5)	(125)		(5)	(125)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(5)	0		(3)	(7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net				0	0
Trading Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period				(8)
Included in income before taxes				(2)
Included in other comprehensive income				0
Purchases/additions				0
Settlements				5
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period				2
Trading derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	2	12			13
Included in income before taxes	1	0			2
Included in other comprehensive income	0	0			0
Purchases/additions	0	0			0
Settlements	(2)	0			(3)
Asset (liability) balance, end of period	1	12		1	12
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	1	0			2
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net					0
Securities Available for Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	1,125	1,088		1,131	1,331
Included in income before taxes	0	0		0	0
Included in other comprehensive income	10	5		11	25
Purchases/additions	350	52		380	56
Settlements	(4)	(4)		(41)	(271)
Asset (liability) balance, end of period	1,481	1,141		1,481	1,141
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	0		0	0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net				0	0
Other Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	200	519		316	277
Included in income before taxes	(23)	(18)		(147)	(94)
Included in other comprehensive income	0	0		0	0
Purchases/additions	3	76		11	394
Settlements	0	0		0	0
Asset (liability) balance, end of period	180	308		180	308
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(23)	(33)		(147)	(109)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ (269)		0	(269)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	$ 0		$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net					$ 0
Trading Assets, Excluding Debt and Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period				8
Included in income before taxes				(2)
Included in other comprehensive income				0
Purchases/additions				0
Settlements				(5)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period				(2)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net				$ 0