Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Components Of Comprehensive Income [Abstract]
Securities available for sale, Unrealized gains (losses) arising during the period, net of tax expense (benefit)	$ 3,649	$ (1,551)	$ 4,302	$ 10,399
Securities available for sale, Reclassification adjustment for gains included in net income, tax expense	110	412	112	3,995
Unrealized gains on derivative instruments arising during the period, net of tax expense	1,709	40	2,353	2,979
Reclassification adjustment on derivative instruments for gains included in net income, tax expense	286	293	854	867
Unrealized gains on pension and postretirement benefit obligations arising during the period, tax expense	$ 183	$ 121	$ 560	$ 355