Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Sep. 30, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (234)	$ 158
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		292	(19)
Reclassification adjustment for gains realized in net income, net of tax	[1]	(18)	(14)
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	5
Total other comprehensive income (loss) for period		278	(28)
Balance at end of period		44	130
Unrealized (losses) gains on derivatives classified as cash flow hedges:
Balance at beginning of period		(170)	(156)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		(48)	(21)
Reclassification adjustment of losses realized in net income, net of tax	[3]	3	2
Total other comprehensive income (loss) for period		(45)	(19)
Balance at end of period		(215)	(175)
Pension and postretirement benefit liability:
Balance at beginning of period		(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	1
Balance at end of period		(3)	(2)
Total accumulated other comprehensive loss at end of period		$ (174)	$ (47)	$ (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.