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
	Mar. 31, 2019	Mar. 31, 2018
Trading derivatives
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period		$ (138)
Included in income before taxes		(10)
Included in other comprehensive income		0
Purchases/additions		0
Settlements		74
Asset (liability) balance, end of period	$ (12)	(54)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period		10
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(118)	(119)
Included in income before taxes	1	0
Included in other comprehensive income	0	0
Purchases/additions	0	0
Settlements	0	0
Asset (liability) balance, end of period	(119)	(119)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(1)	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		139
Included in income before taxes		(10)
Included in other comprehensive income		0
Purchases/additions		0
Settlements		(74)
Asset (liability) balance, end of period	12	55
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period		(10)
Securities Available for Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	1,331	1,600
Included in income before taxes	0	0
Included in other comprehensive income	11	(16)
Purchases/additions	2	72
Settlements	(76)	(150)
Asset (liability) balance, end of period	1,268	1,506
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	0
Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	277	65
Included in income before taxes	(31)	7
Included in other comprehensive income	0	0
Purchases/additions	140	34
Settlements	0	0
Asset (liability) balance, end of period	386	106
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(31)	$ 7
Trading securities
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