Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Parenthetical) (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
Components Of Comprehensive Income [Abstract]
Unrealized gains (losses) on securities, Arising during the period, tax (benefit) expense	$ (2,635)	$ 1,061	$ (8,835)	$ 3,112
Unrealized gains (losses) on securities, Reclassification adjustment for (losses) gains included in net income, tax expense	2	(65)	260	247
Unrealized gains (losses) on derivative instruments, tax benefit	239		(2,215)
Unrealized gains associated with pension and postretirement benefits, tax benefit	$ 194	$ 161	$ 569	$ 461