Investment Securities - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Weighted-average maturity of available-for-sale investment securities	4 years 4 months 24 days	4 years 3 months 18 days
Weighted-average yields of available-for-sale investment securities	2.19%	2.32%
Weighted average maturity of held-to-maturity investment securities	4 years	4 years
Weighted-average yields of held-to-maturity investment securities	1.86%	1.92%
Fair value of securities pledged	$ 13,500	$ 12,600
Fair value of securities pledged as collateral where counterparty has right to repledge or resell	$ 1,100	$ 856