Securities - Securities with Unrealized Losses Aggregated by Investment Category and Length of Time that Individual Securities have been in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 106,735	$ 196,901
Less than 12 Months, Unrealized Loss	(771)	(2,419)
12 Months or More, Fair Value	134,831	64,882
12 Months or More, Unrealized Loss	(3,428)	(3,120)
Total, Fair Value	241,566	261,783
Total, Unrealized Loss	(4,199)	(5,539)
U.S. Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	55,696	90,380
Less than 12 Months, Unrealized Loss	(540)	(579)
12 Months or More, Fair Value	34,754	0
12 Months or More, Unrealized Loss	(368)	0
Total, Fair Value	90,450	90,380
Total, Unrealized Loss	(908)	(579)
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	15,890	32,353
Less than 12 Months, Unrealized Loss	(69)	(448)
12 Months or More, Fair Value	4,104	264
12 Months or More, Unrealized Loss	(245)	(201)
Total, Fair Value	19,994	32,617
Total, Unrealized Loss	(314)	(649)
Residential and Multi-Family Mortgage [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	30,144	65,598
Less than 12 Months, Unrealized Loss	(153)	(1,255)
12 Months or More, Fair Value	63,699	34,611
12 Months or More, Unrealized Loss	(1,729)	(1,127)
Total, Fair Value	93,843	100,209
Total, Unrealized Loss	(1,882)	(2,382)
Corporate Notes and Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5,005	2,089
Less than 12 Months, Unrealized Loss	(9)	(11)
12 Months or More, Fair Value	9,042	8,476
12 Months or More, Unrealized Loss	(453)	(951)
Total, Fair Value	14,047	10,565
Total, Unrealized Loss	(462)	(962)
Pooled SBA [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	6,481
Less than 12 Months, Unrealized Loss	0	(126)
12 Months or More, Fair Value	22,270	20,560
12 Months or More, Unrealized Loss	(575)	(792)
Total, Fair Value	22,270	27,041
Total, Unrealized Loss	(575)	(918)
Other Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Loss	0	0
12 Months or More, Fair Value	962	971
12 Months or More, Unrealized Loss	(58)	(49)
Total, Fair Value	962	971
Total, Unrealized Loss	$ (58)	$ (49)