Derivative Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Basis adjustments for active fair value hedges which increased (decreased) the carrying amount of our debt	$ (4)	$ 2
Amortization of basis adjustments related to terminated and/or re-designated fair value hedge relationships of our debt	8	14
Basis adjustments for active fair value hedges which increased (decreased) the carrying amount of available for sale securities	541	775
Accumulated unamortized basis adjustment for active fair value hedges which increased (decreased) the carrying amount of our available for sale securities	458	(84)
Amortization of basis adjustments related to terminated and/or re-designated cash flow hedge relationships	6	10
Next twelve months, reclassification of remaining losses to earnings resulting from cash flow hedges	11
Aggregate fair value of all derivative instruments with credit-risk-related contingent	7,006	5,614
Collateral already posted aggregate fair value	6,136	5,059
Subordinated Debt [Member]
Derivative [Line Items]
Accumulated unamortized basis adjustment for terminated and/or re-designated fair value hedges of our debt	$ 21	$ 33