Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment [Line Items]
Available-for-sale securities transferred to held-to-maturity securities		$ 536,010,000
Unrealized net loss on securities transferred from available-for-sale to held-to-maturity, Gross		13,300,000
Realized gains on the sale of securities	1,031,000	46,000	226,000
Realized losses on sale of investment securities available for sale	0	0	0
Corporate debt securities, callable, amortized cost	55,000,000
Corporate debt securities, callable, estimated fair value	45,300,000
Estimated fair value of securities	360,388,000	347,238,000
Floating-rate securities, purchased period	1998
Floating-rate debt securities, fixed interest rate period	Over 90-day LIBOR
Principal maturity year	2028 or prior if called by the issuer
Reverse Repurchase Agreements [Member]
Investment [Line Items]
Estimated fair value of securities	$ 70,954,000	$ 78,512,000