Derivative Financial Instruments - Additional Information (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
Derivative [Line Items]
Basis adjustments for active fair value hedges which increased (decreased) the carrying amount of our debt (less than $1 million for the 3 and 6 months ended June 30, 2015)	$ (12)	$ (1)	$ (12)	$ (1)
Amortization of basis adjustments related to terminated and/or re-designated fair value hedge relationships of our debt	2	2	3	5
Basis adjustments for active fair value hedges which increased (decreased) the carrying amount of available for sale securities	(397)	162	(178)	338
Accumulated unamortized basis adjustment for active fair value hedges which increased (decreased) the carrying amount of our available for sale securities	219		219		$ 458
Amortization of basis adjustments related to terminated and/or re-designated cash flow hedge relationships	2	$ 2	5	$ 3
Next twelve months, reclassification of remaining losses to earnings resulting from cash flow hedges			9
Aggregate fair value of all derivative instruments with credit-risk-related contingent	6,165		6,165		7,006
Collateral already posted aggregate fair value	5,017		5,017		6,136
Subordinated Debt [Member]
Derivative [Line Items]
Accumulated unamortized basis adjustment for terminated and/or re-designated fair value hedges of our debt	$ 5		$ 5		$ 21