Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	$ 40,304	$ 225,524
Fair value more than 12 months	30,384	42,380
Total fair value	70,688	267,904
Unrealized losses less than 12 months	(481)	(2,632)
Unrealized losses more than 12 months	(208)	(980)
Total unrealized losses	(689)	(3,613)
U.S. Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	1,998	5,468
Total fair value	1,998	5,468
Unrealized losses less than 12 months	(2)	(17)
Total unrealized losses	(2)	(17)
State and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	1,931	17,353
Fair value more than 12 months	1,812	446
Total fair value	3,743	17,799
Unrealized losses less than 12 months	(186)	(280)
Unrealized losses more than 12 months	(4)	(6)
Total unrealized losses	(190)	(286)
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	6,684	89,459
Fair value more than 12 months	24,739	25,428
Total fair value	31,423	114,887
Unrealized losses less than 12 months	(54)	(1,123)
Unrealized losses more than 12 months	(156)	(655)
Total unrealized losses	(210)	(1,779)
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	29,691	113,244
Fair value more than 12 months	3,833	16,506
Total fair value	33,524	129,750
Unrealized losses less than 12 months	(239)	(1,212)
Unrealized losses more than 12 months	(48)	(319)
Total unrealized losses	$ (287)	$ (1,531)