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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	$ (169)	(186)	$ (166)	$ (223)
Included in income before taxes	(17)	(10)	(57)	(12)
Included in other comprehensive income	0	0	0	0
Purchases/additions	0	0	0	0
Settlements	24	20	61	59
Transfers in (out) of level 3			0	0
Asset (liability) balance, end of period	(162)	(176)	(162)	(176)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(17)	(10)	(57)	(12)
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	(118)	(124)	(121)	(114)
Included in income before taxes	(1)	0	2	(10)
Included in other comprehensive income	0	0	0	0
Purchases/additions	0	0	0	0
Settlements	0		0	0
Transfers in (out) of level 3			0	0
Asset (liability) balance, end of period	(119)	(124)	(119)	(124)
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	(1)	0	2	(10)
Trading derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	171	187	168	228
Included in income before taxes	17	11	58	12
Included in other comprehensive income	0	0	0	0
Purchases/additions	0	0	0	0
Settlements	(24)	(19)	(62)	(61)
Transfers in (out) of level 3			0	0
Asset (liability) balance, end of period	164	179	164	179
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	17	11	58	12
Securities Available for Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset (liability) balance, beginning of period	1,572	1,607	1,638	1,603
Included in income before taxes	0	0	0	0
Included in other comprehensive income	3	(3)	15	2
Purchases/additions	4	1	6	81
Settlements	(10)	(5)	(90)	(86)
Transfers in (out) of level 3			0	0
Asset (liability) balance, end of period	1,569	1,600	1,569	1,600
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	49	15	24	1
Included in income before taxes	(1)	0	(3)	0
Included in other comprehensive income	0	0	0	0
Purchases/additions	9	2	36	3
Settlements	0	0	0	0
Transfers in (out) of level 3			0	13
Asset (liability) balance, end of period	57	17	57	17
Changes in unrealized gains (losses) included in income before taxes for assets and liabilities still held at end of period	$ (1)	$ 0	$ (3)	$ 0