Investment Securities - Amount of Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	$ 14,847	$ 12,078
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(12)	(9)
Twelve Months or Longer, Fair Value	0	4,953
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(55)
Total, Fair Value	14,847	17,031
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(12)	(64)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	305,951	159,393
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7,470)	(3,382)
Twelve Months or Longer, Fair Value	36,069	24,996
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,981)	(466)
Total, Fair Value	342,020	184,389
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9,451)	(3,848)
Corporate Bonds [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	9,779	12,078
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(9)	(9)
Twelve Months or Longer, Fair Value	0	4,953
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(55)
Total, Fair Value	9,779	17,031
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9)	(64)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	58,535	72,234
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,333)	(2,941)
Twelve Months or Longer, Fair Value	33,104	10,669
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,896)	(253)
Total, Fair Value	91,639	82,903
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,229)	(3,194)
U.S. Treasuries [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		0
Twelve Months or Longer, Fair Value		4,887
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(91)
Total, Fair Value		4,887
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		(91)
U.S. Government Corporations and Agencies [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	11,850	72,157
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19)	(379)
Twelve Months or Longer, Fair Value	0	4,972
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(28)
Total, Fair Value	11,850	77,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(19)	(407)
State and Political Subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	40,771	10,251
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(610)	(49)
Twelve Months or Longer, Fair Value	423	1,335
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6)	(12)
Total, Fair Value	41,194	11,586
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(616)	(61)
Residential Mortgage-Backed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)
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	5,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	192,782	4,751
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5,482)	(13)
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	192,782	4,751
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,482)	(13)
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	2,013	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(26)	0
Twelve Months or Longer, Fair Value	2,542	3,133
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(79)	(82)
Total, Fair Value	4,555	3,133
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (105)	$ (82)