Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016		Sep. 30, 2015		Dec. 31, 2015	Sep. 30, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ 252	$ (83)	$ (234)		$ 158
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		(108)	7	407		(210)
Reclassification adjustment for gains realized in net income, net of tax	[1]	(10)	(7)	(51)		(43)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income, net of tax	[2]	6	5	18		17
Total other comprehensive income (loss) for period		(112)	5	374		(236)
Balance at end of period		140	(78)	140		(78)
Unrealized (losses) gains on derivatives classified as cash flow hedges:
Balance at beginning of period		(226)	(144)	(170)		(156)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		6	(34)	(56)		(25)
Reclassification adjustment of losses realized in net income, net of tax		3	2	9	[3]	5	[3]
Total other comprehensive income (loss) for period		9	(32)	(47)		(20)
Balance at end of period		(217)	(176)	(217)		(176)
Pension and postretirement benefit liability:
Balance at beginning of period		(3)	(2)	(3)		(3)
Other comprehensive income for period:
Change in unfunded pension and postretirement liability, net of tax				0		1
Total other comprehensive income for period		0	0	0		1
Balance at end of period		(3)	(2)	(3)		(2)
Total accumulated other comprehensive loss at end of period		$ (80)	$ (256)	$ (80)		$ (256)		$ (407)
AOCI included in pretax unrealized losses									$ 234

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