Accumulated Other Comprehensive Income (Loss) - (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Sep. 30, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ 130	$ 112	$ 158	$ (18)
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		(198)	171	(217)	315
Reclassification adjustment for gains realized in net income, net of tax	[1]	(22)	4	(36)	(10)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income, net of tax	[2]	7	0	12	0
Total other comprehensive income (loss) for period		(213)	175	(241)	305
Balance at end of period		(83)	287	(83)	287
Unrealized losses on other-than-temporarily impaired debt securities held-to-maturity:
Balance at beginning of period		0	(59)	0	(60)
Other comprehensive income for period:
Reclassification adjustment related to the accretion of unrealized other-than-temporary impairment, net of tax	[3]	0	2	0	3
Total other comprehensive income for period		0	2	0	3
Balance at end of period		0	(57)	0	(57)
Unrealized (losses) gains on derivatives classified as cash flow hedges:
Balance at beginning of period		(175)	(103)	(156)	(83)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax		30	(16)	9	(37)
Reclassification adjustment of losses realized in net income, net of tax	[4]	1	1	3	2
Total other comprehensive income (loss) for period		31	(15)	12	(35)
Balance at end of period		(144)	(118)	(144)	(118)
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		$ (229)	$ 114	$ (229)	$ 114	$ (1)
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 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.