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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, end of period	$ 12		$ 12
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	(12)	(49)		$ (138)
Included in income before taxes	0	7		1
Included in other comprehensive income	0	0		0
Purchases/additions	0	0		0
Settlements	0	13		96
Asset (liability) balance, end of period	(12)	(43)	(12)	(43)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	(7)		(1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(125)	(119)	(118)	(119)
Included in income before taxes	0	0	7	0
Included in other comprehensive income	0	0	0	0
Purchases/additions	0	0	0	0
Settlements	0		0	0
Asset (liability) balance, end of period	(125)	(119)	(125)	(119)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	0	(7)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Trading Liabilities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period			(13)
Included in income before taxes			2
Included in other comprehensive income			0
Purchases/additions			0
Settlements			3
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	12	50		139
Included in income before taxes	0	6		0
Included in other comprehensive income	0	0		0
Purchases/additions	0	0		0
Settlements	0	(13)		(96)
Asset (liability) balance, end of period	12	43	12	43
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	6		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
Securities Available for Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	1,088	1,495	1,331	1,600
Included in income before taxes	0	0	0	0
Included in other comprehensive income	5	(1)	25	(18)
Purchases/additions	52	52	56	125
Settlements	(4)	(121)	(271)	(282)
Asset (liability) balance, end of period	1,141	1,425	1,141	1,425
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
Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	519	149	277	65
Included in income before taxes	(18)	2	(94)	11
Included in other comprehensive income	0	0	0	0
Purchases/additions	76	25	394	100
Settlements	0	0	0	0
Asset (liability) balance, end of period	308	176	308	176
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(33)	2	(109)	11
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ (269)	$ 0	(269)	$ 0
Trading Assets, Excluding Debt and Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period			13
Included in income before taxes			2
Included in other comprehensive income			0
Purchases/additions			0
Settlements			(3)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period			$ 2