Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	$ 298,408	$ 425,635
Fair value more than 12 months	35,317	10,199
Total fair value	333,725	435,834
Unrealized losses less than 12 months	(3,976)	(9,413)
Unrealized losses more than 12 months	(729)	(252)
Total unrealized losses	(4,705)	(9,665)
U.S. Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	13,846	6,987
Total fair value	13,846	6,987
Unrealized losses less than 12 months	(58)	(64)
Total unrealized losses	(58)	(64)
State and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	59,252	142,466
Fair value more than 12 months	5,673
Total fair value	64,925	142,466
Unrealized losses less than 12 months	(1,266)	(3,544)
Unrealized losses more than 12 months	(171)
Total unrealized losses	(1,437)	(3,544)
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	79,153	112,414
Fair value more than 12 months	25,382	10,199
Total fair value	104,535	122,613
Unrealized losses less than 12 months	(889)	(1,918)
Unrealized losses more than 12 months	(475)	(252)
Total unrealized losses	(1,364)	(2,170)
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	144,288	163,768
Fair value more than 12 months	4,262
Total fair value	148,550	163,768
Unrealized losses less than 12 months	(1,760)	(3,887)
Unrealized losses more than 12 months	(83)
Total unrealized losses	(1,843)	$ (3,887)
Private Labeled Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	1,869
Total fair value	1,869
Unrealized losses less than 12 months	(3)
Total unrealized losses	$ (3)