Investment Securities - Amount of Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	$ 22,818	$ 14,847
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(31)	(12)
Twelve Months or Longer, Fair Value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	22,818	14,847
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(31)	(12)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	274,132	305,950
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4,824)	(7,470)
Twelve Months or Longer, Fair Value	37,122	36,069
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,951)	(1,981)
Total, Fair Value	311,254	342,019
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(6,775)	(9,451)
U.S. Government Corporations and Agencies [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	22,146	11,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(31)	(19)
Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	22,146	11,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(31)	(19)
State and Political Subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	13,306	40,771
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(74)	(610)
Twelve Months or Longer, Fair Value	1,706	423
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8)	(6)
Total, Fair Value	15,012	41,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(82)	(616)
Residential Mortgage-Backed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	12,820	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(29)	(3)
Twelve Months or Longer, Fair Value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	12,820	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(29)	(3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	169,466	192,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,042)	(5,482)
Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	169,466	192,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,042)	(5,482)
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	1,763	2,012
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(26)
Twelve Months or Longer, Fair Value	2,298	2,542
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(61)	(79)
Total, Fair Value	4,061	4,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(62)	(105)
Corporate Bonds [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	9,998	9,779
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	(9)
Twelve Months or Longer, Fair Value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	9,998	9,779
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)	(9)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	67,448	58,535
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,675)	(1,333)
Twelve Months or Longer, Fair Value	33,118	33,104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,882)	(1,896)
Total, Fair Value	100,566	91,639
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,557)	$ (3,229)
Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)
Twelve Months or Longer, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, Fair Value	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1)