Accumulated Other Comprehensive Income (Loss) - (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016		Jun. 30, 2015		Dec. 31, 2015	Sep. 30, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ 44	$ 130	$ (234)		$ 158
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		223	(198)	515		(217)
Reclassification adjustment for gains realized in net income, net of tax	[1]	(23)	(22)	(41)		(36)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income, net of tax	[2]	8	7	12		12
Total other comprehensive income (loss) for period		208	(213)	486		(241)
Balance at end of period		252	(83)	252		(83)
Unrealized (losses) gains on derivatives classified as cash flow hedges:
Balance at beginning of period		(215)	(175)	(170)		(156)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		(14)	30	(62)		9
Reclassification adjustment of losses realized in net income, net of tax		3	1	6	[3]	3	[3]
Total other comprehensive income (loss) for period		(11)	31	(56)		12
Balance at end of period		(226)	(144)	(226)		(144)
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 income (loss) at end of period		$ 23	$ (229)	$ 23		$ (229)		$ (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 (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] Amount reclassified to net income is included in interest income (expense) in our consolidated statement of income (loss).