Accumulated Other Comprehensive Income (Loss) - (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (354)	$ 44	$ (461)	$ (234)
Other comprehensive income for period:
Net unrealized gains arising during period, net of tax		118	223	224	515
Reclassification adjustment for gains realized in net income, net of tax	[1]	(12)	(23)	(15)	(41)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income, net of tax	[2]	4	8	8	12
Total other comprehensive income for period		110	208	217	486
Balance at end of period		(244)	252	(244)	252
Unrealized gains (losses) on fair value option liabilities attributable to credit spread:
Balance at beginning of period		95	0	174	0
Cumulative effect adjustment to initially apply new accounting guidance for financial liabilities measured under the fair value option, net of tax	[3]					$ 174	$ 0
Other comprehensive income (loss) for period:
Net unrealized losses arising during the period, net of tax		(9)	0	(88)	0
Total other comprehensive loss for period		(9)	0	(88)	0
Balance at end of period		86	0	86	0
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period		(156)	(215)	(157)	(170)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		(5)	(14)	(6)	(62)
Reclassification adjustment of losses realized in net income, net of tax	[4]	2	3	4	6
Total other comprehensive loss for period		(3)	(11)	(2)	(56)
Balance at end of period		(159)	(226)	(159)	(226)
Pension and postretirement benefit liability:
Balance at beginning of period		0	(3)	0	(3)
Balance at end of period		0	(3)	0	(3)
AOCI included in pretax unrealized losses								$ 234
Total accumulated other comprehensive income (loss) at end of period		$ (317)	$ 23	(317)	23	$ (618)
As Previously Reported [Member]
Unrealized gains (losses) on fair value option liabilities attributable to credit spread:
Balance at beginning of period				$ 0	$ 0

[1] Amount reclassified to net income is included in other securities gains, net in our consolidated statement of income (loss).

[2] Amount amortized to net income is included in interest income in our consolidated statement of income (loss). During 2014, we transferred securities from available-for-sale to held-to-maturity. At the date of transfer, AOCI included net pretax unrealized losses of $234 million related to the transferred securities which will be amortized over the remaining contractual life of each security as an adjustment of yield in a manner consistent with the amortization of any premium or discount.

[3] See Note 19, "New Accounting Pronouncements," for additional discussion.

[4] Amount reclassified to net income is included in interest income (expense) in our consolidated statement of income (loss).