Change in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive 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
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 7,097	$ 10,256	$ 12,943	$ 11,202
Other comprehensive income (loss) before reclassifications	(19,316)	7,291	(24,143)	7,608
Less: Amounts reclassified from accumulated other comprehensive loss	(562)	(8,252)	(1,581)	(9,515)
Net current-period other comprehensive loss	(19,878)	(961)	(25,724)	(1,907)
Ending Balance	(12,781)	9,295	(12,781)	9,295
Net Unrealized Losses on Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(472)	(472)	(472)	(472)
Other comprehensive income (loss) before reclassifications
Less: Amounts reclassified from accumulated other comprehensive loss
Net current-period other comprehensive loss
Ending Balance	(472)	(472)	(472)	(472)
Net Unrealized Gains on Investment Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	7,569	10,728	13,415	11,674
Other comprehensive income (loss) before reclassifications	(19,316)	7,291	(24,143)	7,608
Less: Amounts reclassified from accumulated other comprehensive loss	(562)	(8,252)	(1,581)	(9,515)
Net current-period other comprehensive loss	(19,878)	(961)	(25,724)	(1,907)
Ending Balance	$ (12,309)	$ 9,767	$ (12,309)	$ 9,767