Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	$ 201,590	$ 425,635
Fair value more than 12 months	138,336	10,199
Total fair value	339,926	435,834
Unrealized losses less than 12 months	(1,612)	(9,413)
Unrealized losses more than 12 months	(2,989)	(252)
Total unrealized losses	(4,601)	(9,665)
U.S. Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	15,233	6,987
Fair value more than 12 months	1,372
Total fair value	16,605	6,987
Unrealized losses less than 12 months	(71)	(64)
Unrealized losses more than 12 months	(43)
Total unrealized losses	(114)	(64)
State and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	41,995	142,466
Fair value more than 12 months	28,127
Total fair value	70,122	142,466
Unrealized losses less than 12 months	(483)	(3,544)
Unrealized losses more than 12 months	(935)
Total unrealized losses	(1,418)	(3,544)
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	61,135	112,414
Fair value more than 12 months	47,213	10,199
Total fair value	108,348	122,613
Unrealized losses less than 12 months	(480)	(1,918)
Unrealized losses more than 12 months	(991)	(252)
Total unrealized losses	(1,471)	(2,170)
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	81,397	163,768
Fair value more than 12 months	61,624
Total fair value	143,021	163,768
Unrealized losses less than 12 months	(567)	(3,887)
Unrealized losses more than 12 months	(1,020)
Total unrealized losses	(1,587)	$ (3,887)
Private Labeled Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	1,830
Total fair value	1,830
Unrealized losses less than 12 months	(11)
Total unrealized losses	$ (11)