Investment Securities - Amount of Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	$ 17,830	$ 14,847
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(12)	(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	17,830	14,847
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(12)	(12)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	204,331	305,950
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,935)	(7,470)
Twelve Months or Longer, Fair Value	79,448	36,069
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,852)	(1,981)
Total, Fair Value	283,779	342,019
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,787)	(9,451)
U.S. Government Corporations and Agencies [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	6,985
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10)
Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, Fair Value	6,985
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(10)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	16,791	11,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(38)	(19)
Twelve Months or Longer, Fair Value	300	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5)	0
Total, Fair Value	17,091	11,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(43)	(19)
State and Political Subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	8,508	40,771
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(35)	(610)
Twelve Months or Longer, Fair Value	4,402	423
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(21)	(6)
Total, Fair Value	12,910	41,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(56)	(616)
Residential Mortgage-Backed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	10,845	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	(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	10,845	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)	(3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	136,652	192,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,955)	(5,482)
Twelve Months or Longer, Fair Value	22,819	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(481)	0
Total, Fair Value	159,471	192,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,436)	(5,482)
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	0	2,012
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(26)
Twelve Months or Longer, Fair Value	2,167	2,542
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(57)	(79)
Total, Fair Value	2,167	4,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(57)	(105)
Corporate Bonds [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value		9,779
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(9)
Twelve Months or Longer, Fair Value		0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Total, Fair Value		9,779
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss		(9)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	42,377	58,535
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(906)	(1,333)
Twelve Months or Longer, Fair Value	49,760	33,104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,288)	(1,896)
Total, Fair Value	92,137	91,639
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,194)	$ (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)