Securities Available for Sale - Additional Information (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
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 11	$ 3	$ 25	$ 3
Available-for-sale Securities, Gross Realized Losses		(3)	12	3	13
Available-for-sale Securities Pledged as Collateral		2,900		2,900		$ 2,600
Available-for-sale Securities	[1]	27,270		27,270		26,770
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		102		102		$ 123
Other Than Temporarily Impaired Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		$ 22	$ 19	$ 22	$ 19

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $0 million and $369 million at September 30, 2015 and December 31, 2014, respectively.