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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
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	$ (54)	(168)	$ (138)	$ (166)
Included in income before taxes	4	12	(6)	40
Included in other comprehensive income	0	0	0	0
Purchases/additions	0	0	0	0
Settlements	9	11	83	37
Asset (liability) balance, end of period	(49)	(169)	(49)	(169)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(4)	(12)	6	(40)
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(119)	(119)	(119)	(121)
Included in income before taxes	0	(1)	0	(3)
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	(119)	(118)	(119)	(118)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	0	1	0	3
Trading derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	55	172	139	168
Included in income before taxes	4	11	(6)	41
Included in other comprehensive income	0	0	0	0
Purchases/additions	0	0	0	0
Settlements	(9)	(12)	(83)	(38)
Asset (liability) balance, end of period	50	171	50	171
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	4	11	(6)	41
Securities Available for Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	1,506	1,569	1,600	1,638
Included in income before taxes	0	0	0	0
Included in other comprehensive income	(1)	9	(17)	12
Purchases/additions	1	1	73	2
Settlements	(11)	(7)	(161)	(80)
Asset (liability) balance, end of period	1,495	1,572	1,495	1,572
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	106	35	65	24
Included in income before taxes	2	(2)	9	(2)
Included in other comprehensive income	0	0	0	0
Purchases/additions	41	16	75	27
Settlements	0	0	0	0
Asset (liability) balance, end of period	149	49	149	49
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	$ 2	$ (2)	$ 9	$ (2)