Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Basis adjustments for active fair value hedges which increased (decreased) the carrying amount of our debt	$ (31)	$ (12)	$ (96)	$ (12)
Amortization of basis adjustments related to terminated and/or re-designated fair value hedge relationships of our debt	1	2	3	3
Basis adjustments for active fair value hedges which increased (decreased) the carrying amount of available for sale securities	(352)	397	(954)	178
Accumulated unamortized basis adjustment for active fair value hedges which increased (decreased) the carrying amount of our available for sale securities	1,395		1,395		$ 439
Amortization of basis adjustments related to terminated and/or re-designated cash flow hedge relationships	5	$ 2	9	$ 5
Next twelve months, reclassification of remaining losses to earnings resulting from cash flow hedges			15
Aggregate fair value of all derivative instruments with credit-risk-related contingent	6,731		6,731		7,139
Collateral already posted aggregate fair value	6,559		6,559		6,283
Long-term Debt [Member]
Derivative [Line Items]
Accumulated unamortized basis adjustment for fair value hedges which increased (decreased) the carrying amount of our debt	$ (87)		$ (87)		$ (6)