Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Components and Related Tax Effects (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]
Unrealized holding gain (loss) on securities available for sale	$ (2,121)	$ 6,759	$ (35,428)	$ 13,270
Unrealized holding gain (loss) on postretirement benefits
Reclassification adjustment for gains realized in income	0	(1,192)	(2,578)	(5,161)
Net unrealized gains (losses)	(2,121)	5,567	(38,006)	8,109
Tax effect		(2,838)		(2,838)
Net of tax amount	$ (2,121)	$ 2,729	$ (38,006)	$ 5,271