Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (461)	$ (234)
Other comprehensive income for period:
Net unrealized gains arising during period, net of tax		106	292
Reclassification adjustment for gains realized in net income, net of tax	[1]	(3)	(18)
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	4
Total other comprehensive income for period		107	278
Balance at end of period		(354)	44
Unrealized gains (losses) on fair value option liabilities attributable to credit spread:
Balance at beginning of period		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) Net of Tax, Unrealized Gains (Losses) on Fair Value Option Liabilities Attributable to Credit Spread [Abstract]
Net unrealized losses arising during the period, net of tax		(79)	0
Total other comprehensive loss for period		(79)	0
Balance at end of period		95	0
Unrealized (losses) gains on derivatives classified as cash flow hedges:
Balance at beginning of period		(157)	(170)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		(1)	(48)
Reclassification adjustment of losses realized in net income, net of tax	[4]	2	3
Total other comprehensive income (loss) for period		1	(45)
Balance at end of period		(156)	(215)
Pension and postretirement benefit liability:
Balance at beginning of period		0	(3)
Balance at end of period		0	(3)
AOCI included in pretax unrealized losses						$ 234
Total accumulated other comprehensive loss at end of period		(415)	(174)	$ (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.

[2] Amount amortized to net income is included in interest income in our consolidated statement of income. 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.