Securities Available for Sale - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 25	$ 28	$ 39
Available-for-sale Securities, Gross Realized Losses		3	42	$ 36
Available-for-sale Securities Pledged as Collateral		3,200	2,600
Available-for-sale Securities	[1]	27,825	26,770
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		94	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		$ 20	$ 16

[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 December 31, 2015 and December 31, 2014, respectively.