Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Parenthetical) (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]
Securities available for sale, Unrealized (losses) gains arising during the period, tax (benefit) expense	$ (3,870)	$ 2,935	$ 652	$ 11,950
Securities available for sale, Reclassification adjustment for gains included in net income, tax expense	5	272	2	3,583
Unrealized (losses) gains on derivative instruments arising during the period, tax (benefit) expense	(860)	1,537	645	2,939
Reclassification adjustment on derivative instruments for gains included in net income, tax expense	285	290	568	575
Unrealized gains on pension and postretirement benefit obligations arising during the period, tax expense	$ 189	$ 117	$ 377	$ 234