Securities Available for Sale - Additional Information (Detail) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities Pledged as Collateral		$ 3,100	$ 2,600
Available-for-sale Securities	[1]	27,113	26,770
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		112	$ 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		$ 0		$ 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 June 30, 2015 and December 31, 2014, respectively.