Accumulated Other Comprehensive Income (Loss) - (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (83)	$ 287	$ 158	$ (18)
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		7	(71)	(210)	243
Reclassification adjustment for gains realized in net income, net of tax	[1]	(7)	(17)	(43)	(26)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income, net of tax	[2]	5	1	17	1
Total other comprehensive income (loss) for period		5	(87)	(236)	218
Balance at end of period		(78)	200	(78)	200
Unrealized losses on other-than-temporarily impaired debt securities held-to-maturity:
Balance at beginning of period		0	(57)	0	(60)
Other comprehensive income for period:
Reclassification adjustment related to the accretion of unrealized other-than-temporary impairment, net of tax	[3]	0	6	0	9
Total other comprehensive income for period		0	6	0	9
Balance at end of period		0	(51)	0	(51)
Unrealized (losses) gains on derivatives classified as cash flow hedges:
Balance at beginning of period		(144)	(118)	(156)	(83)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		(34)	(6)	(25)	(42)
Reclassification adjustment of losses realized in net income, net of tax	[4]	2	1	5	2
Total other comprehensive income (loss) for period		(32)	(5)	(20)	(40)
Balance at end of period		(176)	(123)	(176)	(123)
Pension and postretirement benefit liability:
Balance at beginning of period		(2)	2	(3)	2
Other comprehensive income for period:
Change in unfunded pension and postretirement liability, net of tax				1	0
Total other comprehensive income for period		0	0	1	0
Balance at end of period		(2)	2	(2)	2
Total accumulated other comprehensive income (loss) at end of period		$ (256)	28	$ (256)	28	$ (1)
AOCI included in pretax unrealized losses			$ 234		$ 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 the third quarter of 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 the carrying value of the debt securities is included in securities held-to-maturity in our consolidated balance sheet.

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