Change in Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 477	$ 365	$ 928	$ 943
Interest income on investment securities	852	814	1,712	1,606
Income before taxes	19,121	19,826	39,334	35,720
Income taxes	(6,887)	(7,101)	(14,211)	(6,084)
Net income	12,234	12,725	25,123	29,636
Salaries, benefits and other compensation	20,345	18,668	41,355	37,142
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	274	166	398	525
Reclassification out of Accumulated Other Comprehensive Income [Member] | Prior Service (Credits) Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, benefits and other compensation	(19)	919	(37)	919
Reclassification out of Accumulated Other Comprehensive Income [Member] | Transition Obligation [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, benefits and other compensation		245		245
Reclassification out of Accumulated Other Comprehensive Income [Member] | Actuarial (Gains) Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	(5)	(1,068)	(10)	(1,068)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before taxes	(24)	96	(47)	96
Income taxes	(9)	(36)	17	(36)
Net income	(15)	60	(30)	60
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Gains on Securities Transactions [Member] | Net Unrealized Gains (Losses) on Investment Securities Available-for-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	477	365	928	943
Income taxes	(181)	(139)	(352)	(358)
Net income	296	$ 226	576	$ 585
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization of Net Unrealized Gains to Income During Period [Member] | Net Unrealized Gains (Losses) on Investment Securities Available-for-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income on investment securities	(157)		(328)
Income taxes	120		120
Net income	$ (37)		$ (208)