Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Components Of Comprehensive Income [Abstract]
Securities available for sale, Unrealized gains (losses) arising during the period, net of tax expense	$ 3,634	$ (3,870)	$ 11,353	$ 652
Securities available for sale, Reclassification adjustment for gains included in net income, tax expense	79	5	104	2
Unrealized gains (losses) on derivative instruments arising during the period, net of tax expense (benefit)	833	(860)	2,527	645
Reclassification adjustment on derivative instruments for gains included in net income, tax expense	191	285	379	568
Unrealized gains on pension and postretirement benefit obligations arising during the period, tax expense	$ 214	$ 189	$ 427	$ 377