Investment Securities - Amount of Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	$ 47,800	$ 14,847
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(288)	(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	47,800	14,847
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(288)	(12)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	74,351	305,950
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,356)	(7,470)
Twelve Months or Longer, Fair Value	233,434	36,069
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6,714)	(1,981)
Total, Fair Value	307,785	342,019
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(8,070)	(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	6,919
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(77)
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	6,919
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(77)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	5,213	11,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(38)	(19)
Twelve Months or Longer, Fair Value	11,749	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(90)	0
Total, Fair Value	16,962	11,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(128)	(19)
State and Political Subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	18,457	40,771
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(91)	(610)
Twelve Months or Longer, Fair Value	6,332	423
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(32)	(6)
Total, Fair Value	24,789	41,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(123)	(616)
Residential Mortgage-Backed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	40,881	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(211)	(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	40,881	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(211)	(3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	32,217	192,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(210)	(5,482)
Twelve Months or Longer, Fair Value	141,371	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3,051)	0
Total, Fair Value	173,588	192,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,261)	(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	2,021	2,542
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(82)	(79)
Total, Fair Value	2,021	4,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(82)	(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	18,464	58,535
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,016)	(1,333)
Twelve Months or Longer, Fair Value	71,957	33,104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3,459)	(1,896)
Total, Fair Value	90,421	91,639
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4,475)	$ (3,229)
Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)
Twelve Months or Longer, Fair Value	4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, Fair Value	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1)