Investment Securities - Investment Securities (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Contractual Maturities of Investment Securities [Abstract]
Federal statutory rate	35.00%
Weighted-average maturity of available-for-sale investment securities	4 years 4 months 24 days	4 years 3 months 18 days
Weighted-average yield 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 yield of held-to-maturity investment securities	1.86%	1.92%