Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 security	Mar. 31, 2014 security	Dec. 31, 2014 security
Schedule of Available-for-sale Securities [Line Items]
Assets backed securities in available for sale portfolio	15		15
Assets backed securities wrapped by monoline insurance company	5		5
Value of asset backed securities backed by insurance company	$ 180		$ 183
Fair value of asset backed securities	208		226
Available-for-sale securities, backed by insurance company, accumulated unrecognized holding loss	27		23
Fair value of financial guarantee	85		89
Held-to-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset backed securities with changes to other than temporary impairment estimates related to credit component	0	1
Increase in credit losses for which an other-than-temporary impairment was previously recognized	0	2
Other assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank Stock	315		108
Federal Reserve Bank Stock	$ 485		$ 483