Other Comprehensive Income (Loss) - Summary of Each Component of Accumulated Other Comprehensive Income (Loss), Net 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
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]
Unrealized gains/losses on Available for Sale Securities, Beginning Balance	$ 2,119		$ 5,082
Other comprehensive loss before reclassification, Unrealized gains/losses on Available for Sale Securities	(31,065)		(33,307)
Amounts reclassified from accumulated other comprehensive income, Unrealized gains/losses on Available for Sale Securities	1,857	3,555	2,578	3,969
Unrealized gains (losses) on securities available for sale, Current Period Change	(32,922)	5,425	(35,885)	2,542
Unrealized gains/losses on Available for Sale Securities, Ending Balance	(30,803)		(30,803)
Postretirement Benefits, Beginning Balance	1,600		1,600
Other comprehensive loss before reclassification, Postretirement Benefits
Amounts reclassified from accumulated other comprehensive income, Postretirement Benefits
Net current period other comprehensive loss, Postretirement Benefits
Postretirement Benefits, Ending Balance	1,600		1,600
Total, Beginning Balance	3,719		6,682
Other comprehensive loss before reclassification, Total	(31,065)		(33,307)
Amounts reclassified from accumulated other comprehensive income, Total	(1,857)		(2,578)
Total other comprehensive income (loss)	(32,922)	5,425	(35,885)	2,542
Total, Ending Balance	$ (29,203)		$ (29,203)