Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	$ 369,191	$ 229,768
Fair value more than 12 months	186,029	193,869
Total fair value	555,220	423,637
Unrealized losses less than 12 months	(9,389)	(4,554)
Unrealized losses more than 12 months	(7,858)	(4,974)
Total unrealized losses	(17,247)	(9,528)
U.S. Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	17,929	15,882
Fair value more than 12 months	2,853	2,870
Total fair value	20,782	18,752
Unrealized losses less than 12 months	(320)	(180)
Unrealized losses more than 12 months	(62)	(45)
Total unrealized losses	(382)	(225)
State and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	155,952	54,312
Fair value more than 12 months	29,869	30,691
Total fair value	185,821	85,003
Unrealized losses less than 12 months	(4,750)	(2,758)
Unrealized losses more than 12 months	(1,537)	(844)
Total unrealized losses	(6,287)	(3,602)
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	60,700	54,006
Fair value more than 12 months	69,729	73,462
Total fair value	130,429	127,468
Unrealized losses less than 12 months	(1,332)	(589)
Unrealized losses more than 12 months	(2,868)	(2,031)
Total unrealized losses	(4,200)	(2,620)
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	133,020	103,926
Fair value more than 12 months	83,578	86,846
Total fair value	216,598	190,772
Unrealized losses less than 12 months	(2,962)	(1,019)
Unrealized losses more than 12 months	(3,391)	(2,054)
Total unrealized losses	(6,353)	(3,073)
Private Labeled Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	1,590	1,642
Total fair value	1,590	1,642
Unrealized losses less than 12 months	(25)	(8)
Total unrealized losses	$ (25)	$ (8)