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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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)	(54)		$ (138)
Included in income before taxes		4		(6)
Included in other comprehensive income		0		0
Purchases/additions		0		0
Settlements		9		83
Asset (liability) balance, end of period	(12)	(49)	(12)	(49)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	(4)		6
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(119)	(119)	(118)	(119)
Included in income before taxes	6	0	7	0
Included in other comprehensive income		0	0	0
Purchases/additions		0	0	0
Settlements			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	(6)	0	(7)	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	55		139
Included in income before taxes		4		(6)
Included in other comprehensive income		0		0
Purchases/additions		0		0
Settlements		(9)		(83)
Asset (liability) balance, end of period	12	50	12	50
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	4		(6)
Securities Available for Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	1,268	1,506	1,331	1,600
Included in income before taxes	0	0	0	0
Included in other comprehensive income	9	(1)	20	(17)
Purchases/additions	2	1	4	73
Settlements	(191)	(11)	(267)	(161)
Asset (liability) balance, end of period	1,088	1,495	1,088	1,495
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	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	386	106	277	65
Included in income before taxes	(45)	2	(76)	9
Included in other comprehensive income	0	0	0	0
Purchases/additions	178	41	318	75
Settlements	0	0	0	0
Asset (liability) balance, end of period	519	149	519	149
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	$ (45)	$ 2	(76)	$ 9
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