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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	$ 1,339	$ 24	$ 1,524	$ 282
Amortization of net loss included in net periodic pension costs	349	292	641	589
Net loss on interest rate swap	(1,866)		(1,866)
Other-than-temporary impairment on equity securities		(6)		(9)
Accretion of prior service cost included in net periodic pension costs	(63)	(64)	(127)	(128)
Income before income taxes	6,367	6,168	13,475	12,377
Tax expense (benefit)	(1,537)	(1,405)	(3,247)	(2,351)
Net of tax	4,830	4,763	10,228	10,026
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Unrealized 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	1,339	24	1,524	282
Other-than-temporary impairment on equity securities		(6)		(9)
Income before income taxes	1,339	18	1,524	273
Tax expense (benefit)	(468)	(7)	(533)	(96)
Net of tax	871	11	991	177
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	(1,866)		(1,866)
Income before income taxes	(1,866)		(1,866)
Tax expense (benefit)	653		653
Net of tax	(1,213)		(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	(349)	(292)	(641)	(589)
Accretion of prior service cost included in net periodic pension costs	63	64	127	128
Income before income taxes	(286)	(228)	(514)	(461)
Tax expense (benefit)	101	80	180	161
Net of tax	$ (185)	$ (148)	$ (334)	$ (300)