Investment Securities - Schedule of Gross Unrealized Losses and Fair Value of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-Sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 305,435	$ 51,798	$ 29,195
12 Months or More, Fair Value	46,004	81,155	99,632
Total Fair Value	351,439	132,953	128,827
Less Than 12 Months, Unrealized Losses	(2,922)	(291)	(100)
12 Months or More, Unrealized Losses	(1,371)	(1,820)	(3,581)
Total Unrealized Losses	(4,293)	(2,111)	(3,681)
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Less Than 12 Months, Fair Value	9,818
12 Months or More, Fair Value	4,928	14,678	14,445
Total Fair Value	14,746	14,678	14,445
Less Than 12 Months, Unrealized Losses	(138)
12 Months or More, Unrealized Losses	(72)	(275)	(505)
Total Unrealized Losses	(210)	(275)	(505)
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Less Than 12 Months, Fair Value	50,294	15,038	4,088
12 Months or More, Fair Value	10,404	19,665	29,203
Total Fair Value	60,698	34,703	33,291
Less Than 12 Months, Unrealized Losses	(680)	(70)	(35)
12 Months or More, Unrealized Losses	(323)	(363)	(828)
Total Unrealized Losses	(1,003)	(433)	(863)
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Less Than 12 Months, Fair Value	7,149
12 Months or More, Fair Value			4,945
Total Fair Value	7,149		4,945
Less Than 12 Months, Unrealized Losses	(58)
12 Months or More, Unrealized Losses			(131)
Total Unrealized Losses	(58)		(131)
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Less Than 12 Months, Fair Value	238,174	36,760	25,107
12 Months or More, Fair Value	30,672	46,812	51,039
Total Fair Value	268,846	83,572	76,146
Less Than 12 Months, Unrealized Losses	(2,046)	(221)	(65)
12 Months or More, Unrealized Losses	(976)	(1,182)	(2,117)
Total Unrealized Losses	$ (3,022)	$ (1,403)	$ (2,182)