Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Realized gains on the sale of securities	$ 0	$ 1,000,000	$ 46,000
Realized losses on sale of investment securities	0	0	$ 0
Corporate debt securities, callable, amortized cost	56,000,000
Corporate debt securities, callable, estimated fair value	47,500,000
Estimated fair value of securities pledged for deposits and other purposes	$ 317,900,000	360,400,000
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 pledged for reverse repurchase agreements	$ 83,200,000	$ 70,900,000