Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost within one year	$ 6,786	$ 7,192
Amortized cost one to five years	46,633	38,197
Amortized cost for five to ten years	15,505	16,807
Amortized cost for after ten years	15,724	17,605
Total amortized cost	84,648	79,801
Total available for sale investment securities, Amortized Cost	455,502	444,062
Within one year, amortized cost	0	0
One to five years, amortized cost	15,360	17,815
Five to ten years, amortized cost	98,023	106,167
After ten years, amortized cost	32,137	28,208
Total amortized cost	145,520	152,190
Total held to maturity investment securities, amortized cost	180,291	187,629
Fair value within one year	6,816	7,232
Fair value for one to five years	47,547	38,894
Fair value for five to ten years	16,047	17,152
Fair value for after ten years	16,178	17,797
Total fair value	86,588	81,075
Investment securities, available for sale	462,476	444,982
Within one year, fair value	0	0
One to five years, fair value	16,126	18,403
five to ten years, fair value	102,861	110,026
After ten years, fair value	33,098	28,976
Total fair value	152,085	157,405
Held-to-maturity, Fair Value	188,093	193,703
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	155,687	157,291
Total held to maturity investment securities, amortized cost	8,731	9,051
Investment securities, available for sale	157,313	156,203
Held-to-maturity, Fair Value	8,867	8,954
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	215,167	206,970
Total held to maturity investment securities, amortized cost	26,040	26,388
Investment securities, available for sale	218,576	207,704
Held-to-maturity, Fair Value	$ 27,141	$ 27,344