Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified Out of Each Component of Accumulated Comprehensive (Loss) Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	$ 0		$ 1,426		$ 557		$ 2,950
Amortization of net loss included in net periodic pension costs	168		320		499		961
Accretion of prior service cost included in net periodic pension costs	(72)		(64)		(216)		(191)
Income before income taxes	8,499		7,439		22,857		20,914
Tax (expense) benefit	(2,264)		(1,400)		(5,816)		(4,647)
Income, Net of Tax Amount	6,235		6,039		17,041		16,267
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	0		1,426		557		2,950
Income before income taxes	0		1,426		557		2,950
Tax (expense) benefit	0		(499)		(195)		(1,032)
Income, Net of Tax Amount	0		927		362		1,918
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Change Related to Derivative Used for Cash Flow Hedge [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net loss on interest rate swap	0		0		0		(1,866)
Income before income taxes	0		0		0		(1,866)
Tax (expense) benefit	0		0		0		653
Income, Net of Tax Amount	0		0		0		(1,213)
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Change Related to Defined Benefit Pension Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss included in net periodic pension costs	(168)	[1]	(320)	[1]	(499)	[1]	(961)	[1]
Accretion of prior service cost included in net periodic pension costs	72	[1]	64	[1]	216	[1]	191	[1]
Income before income taxes	(96)		(256)		(283)		(770)
Tax (expense) benefit	33		90		98		270
Income, Net of Tax Amount	$ (63)		$ (166)		$ (185)		$ (500)

[1] These accumulated other comprehensive income (loss) components are included in the computation of net periodic pension cost. (See Note 7—Retirement Plans and Other Postretirement Benefits for additional details.)