Investment Securities - Gross Unrealized Losses on Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 117,499	$ 6,774
Less than 12 months, Unrealized Loss	(883)	(25)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	117,499	6,774
Total Unrealized Loss	(883)	(25)
Less than 12 months, Fair Value, held to maturity	136,397	1,935
Less than 12 months, Unrealized Loss, held to maturity	(797)	(27)
12 months or more, Fair Value, held to maturity	1,090	58,146
12 months or more, Unrealized Loss, held to maturity	(130)	(805)
Total Fair Value, held to maturity	137,487	60,081
Total Unrealized Loss, held to maturity	(927)	(832)
Obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	94,392	6,774
Less than 12 months, Unrealized Loss	(451)	(25)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	94,392	6,774
Total Unrealized Loss	(451)	(25)
Less than 12 months, Fair Value, held to maturity	131,022	335
Less than 12 months, Unrealized Loss, held to maturity	(754)	(1)
12 months or more, Fair Value, held to maturity		56,288
12 months or more, Unrealized Loss, held to maturity		(676)
Total Fair Value, held to maturity	131,022	56,623
Total Unrealized Loss, held to maturity	(754)	(677)
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	23,107
Less than 12 months, Unrealized Loss	(432)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	23,107
Total Unrealized Loss	(432)
Less than 12 months, Fair Value, held to maturity	5,375	1,600
Less than 12 months, Unrealized Loss, held to maturity	(43)	(26)
12 months or more, Fair Value, held to maturity	1,090	1,858
12 months or more, Unrealized Loss, held to maturity	(130)	(129)
Total Fair Value, held to maturity	6,465	3,458
Total Unrealized Loss, held to maturity	(173)	(155)
Marketable equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	$ 0	0
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or more, Fair Value		0
12 months or more, Unrealized Loss		$ 0