Investment Securities - Amount of Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 0	$ 15,036
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(27)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,987	4,987
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(22)	(25)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	4,987	20,023
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(22)	(52)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	64,889	76,626
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,326)	(471)
Twelve Months or Longer, Fair Value	25,383	95,818
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(294)	(1,057)
Total, Fair Value	90,272	172,444
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,620)	(1,528)
U.S. Treasuries [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Twelve Months or Longer, Fair Value	4,945	4,845
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(32)	(127)
Total, Fair Value	4,945	4,845
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(32)	(127)
U.S. Government Corporations and Agencies [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	3,004	39,607
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8)	(80)
Twelve Months or Longer, Fair Value	4,995	62,140
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5)	(452)
Total, Fair Value	7,999	101,747
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(13)	(532)
State and Political Subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	11,924	10,246
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(90)	(31)
Twelve Months or Longer, Fair Value	1,340	9,303
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9)	(106)
Total, Fair Value	13,264	19,549
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(99)	(137)
Residential Mortgage-Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value		4,824
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(32)
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		4,824
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		(32)
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Twelve Months or Longer, Fair Value	3,304	3,725
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(49)	(85)
Total, Fair Value	3,304	3,725
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(49)	(85)
Corporate Bonds [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	15,036
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(27)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,987	4,987
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(22)	(25)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	4,987	20,023
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(22)	(52)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than Twelve Months, Fair Value	49,961	21,949
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,228)	(328)
Twelve Months or Longer, Fair Value	10,799	15,805
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(199)	(287)
Total, Fair Value	60,760	37,754
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (2,427)	$ (615)