Change in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 12,943	$ 11,202
Other comprehensive income (loss) before reclassifications	(4,827)	316
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(1,019)	(1,262)
Net current-period other comprehensive income (loss)	(5,846)	(946)
Ending Balance	7,097	10,256
Net Unrealized Losses on Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(472)	(472)
Other comprehensive income (loss) before reclassifications
Less: Amounts reclassified from accumulated other comprehensive income (loss)
Net current-period other comprehensive income (loss)
Ending Balance	(472)	(472)
Net Unrealized Gains on Investment Securities Available For Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	13,415	11,674
Other comprehensive income (loss) before reclassifications	(4,827)	316
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(1,019)	(1,262)
Net current-period other comprehensive income (loss)	(5,846)	(946)
Ending Balance	$ 7,569	$ 10,728