Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Components Of Comprehensive Income [Abstract]
Net income	$ 40,131	$ 34,831	$ 80,474	$ 69,355
Securities available for sale, Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(3,870), $2,935, $652 and $11,950	(7,188)	5,451	1,212	22,193
Securities available for sale, Reclassification adjustment for gains included in net income, net of tax expense of $5, $272, $2 and $3,583	(9)	(504)	(3)	(6,654)
Unrealized (losses) gains on derivative instruments arising during the period, net of tax(benefit) expense of $(860), $1,537, $645 and $2,939	(1,597)	2,855	1,196	5,458
Reclassification adjustment on derivative instruments for gains included in net income, net of tax expense of $285, $290, $568 and $575	(529)	(538)	(1,055)	(1,067)
Unrealized gains on pension and postretirement benefit obligations arising during the period, net of tax expense of $189, $117, $377 and $234	350	218	700	435
Other comprehensive (loss) income	(8,973)	7,482	2,050	20,365
Comprehensive income	$ 31,158	$ 42,313	$ 82,524	$ 89,720