Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (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]
Net income	$ 31,634	$ 30,743	$ 89,365	$ 81,455
Unrealized gains (losses) on securities, Arising during the period, net of tax (benefit) expense of $(2,635), $1,061, $(8,835) and $3,112	(4,894)	1,970	(16,408)	5,779
Unrealized gains (losses) on securities, Less: reclassification adjustment for (losses) gains included in net income, net of tax (benefit) expense of $2, $(65), $260 and $247	(3)	120	(483)	(459)
Unrealized gains (losses) on derivative instruments, net of tax expense (benefit) of $239 and $(2,215)	443		(4,113)
Unrealized gains associated with pension and postretirement benefits, net of tax expense of $194, $161, $569 and $461	360	299	1,057	856
Other comprehensive income (loss)	(4,094)	2,389	(19,947)	6,176
Comprehensive income	$ 27,540	$ 33,132	$ 69,418	$ 87,631