Securities Available-for-Sale - Fair Value and Age of Gross Unrealized Losses by Investment Category (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014 security	Dec. 31, 2013 security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions	22	41
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 191,970	$ 274,942
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses	(754)	(6,913)
Available-for-sale, Securities, 12 Months or More, Number of Positions	13	2
Available-for-sale Securities, 12 Months or More, Fair Value	103,133	10,088
Available-for-sale Securities, 12 Months or More, Unrealized Losses	(2,361)	(863)
Number of debt securities on which unrealized losses were primarily attributable to changes in interest	35	43
Available-for-sale Securities, Fair Value, Total	295,103	285,030
Available-for-sale Securities, Unrealized Losses, Total	(3,115)	(7,776)
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions	3
Available-for-sale Securities, Less Than 12 Months, Fair Value	14,803
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses	(60)
Available-for-sale, Securities, 12 Months or More, Number of Positions
Available-for-sale Securities, 12 Months or More, Fair Value
Available-for-sale Securities, 12 Months or More, Unrealized Losses
Available-for-sale, Securities, Number of Positions	3
Available-for-sale Securities, Fair Value, Total	14,803
Available-for-sale Securities, Unrealized Losses, Total	(60)
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions	9	16
Available-for-sale Securities, Less Than 12 Months, Fair Value	83,654	126,017
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses	(377)	(2,581)
Available-for-sale, Securities, 12 Months or More, Number of Positions	8
Available-for-sale Securities, 12 Months or More, Fair Value	62,754
Available-for-sale Securities, 12 Months or More, Unrealized Losses	(1,401)
Available-for-sale, Securities, Number of Positions	17	16
Available-for-sale Securities, Fair Value, Total	146,408	126,017
Available-for-sale Securities, Unrealized Losses, Total	(1,778)	(2,581)
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions	6	3
Available-for-sale Securities, Less Than 12 Months, Fair Value	71,273	39,522
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses	(217)	(603)
Available-for-sale, Securities, Number of Positions	6	3
Available-for-sale Securities, Fair Value, Total	71,273	39,522
Available-for-sale Securities, Unrealized Losses, Total	(217)	(603)
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions		2
Available-for-sale Securities, Less Than 12 Months, Fair Value		22,822
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses		(685)
Available-for-sale, Securities, 12 Months or More, Number of Positions	1
Available-for-sale Securities, 12 Months or More, Fair Value	9,118
Available-for-sale Securities, 12 Months or More, Unrealized Losses	(253)
Available-for-sale, Securities, Number of Positions	1	2
Available-for-sale Securities, Fair Value, Total	9,118	22,822
Available-for-sale Securities, Unrealized Losses, Total	(253)	(685)
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions	1	4
Available-for-sale Securities, Less Than 12 Months, Fair Value	9,927	39,052
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses	(97)	(1,305)
Available-for-sale, Securities, 12 Months or More, Number of Positions	2
Available-for-sale Securities, 12 Months or More, Fair Value	20,546
Available-for-sale Securities, 12 Months or More, Unrealized Losses	(570)
Available-for-sale, Securities, Number of Positions	3	4
Available-for-sale Securities, Fair Value, Total	30,473	39,052
Available-for-sale Securities, Unrealized Losses, Total	(667)	(1,305)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions	3	16
Available-for-sale Securities, Less Than 12 Months, Fair Value	12,313	47,529
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses	(3)	(1,739)
Available-for-sale, Securities, 12 Months or More, Number of Positions	2	2
Available-for-sale Securities, 12 Months or More, Fair Value	10,715	10,088
Available-for-sale Securities, 12 Months or More, Unrealized Losses	(137)	(863)
Available-for-sale, Securities, Number of Positions	5	18
Available-for-sale Securities, Fair Value, Total	23,028	57,617
Available-for-sale Securities, Unrealized Losses, Total	(140)	(2,602)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Less Than 12 Months, Number of Positions		41
Available-for-sale Securities, Less Than 12 Months, Fair Value		274,942
Available-for-sale Securities, Less Than 12 Months, Unrealized Losses		(6,913)
Available-for-sale, Securities, 12 Months or More, Number of Positions		2
Available-for-sale Securities, 12 Months or More, Fair Value		10,088
Available-for-sale Securities, 12 Months or More, Unrealized Losses		(863)
Available-for-sale, Securities, Number of Positions		43
Available-for-sale Securities, Fair Value, Total		285,030
Available-for-sale Securities, Unrealized Losses, Total		(7,776)
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities, Number of Positions
Available-for-sale Securities, Fair Value, Total
Available-for-sale Securities, Unrealized Losses, Total