Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Allowance for loan losses	$ 23,778	$ 24,160
Post-retirement benefit	10,869	10,658
Deferred compensation	2,825	3,009
Intangibles	481	499
Purchase accounting adjustments	1,464	387
Depreciation	3,868	3,963
SERP	941	949
ESOP	3,254	3,264
Stock-based compensation	5,791	5,734
Non-accrual interest	4,915	5,202
State NOL	139	430
Federal NOL	1,058	1,376
Pension liability adjustments	4,311	5,178
Other	2,048	1,073
Total gross deferred tax assets	65,742	65,882
Valuation Reserve	0	(242)
Deferred tax liabilities:
Pension expense	10,124	9,925
Deferred loan costs	5,156	4,089
Investment securities, principally due to accretion of discounts	146	311
Originated mortgage servicing rights	347	395
Unrealized gain on securities	2,646	5,191
Total gross deferred tax liabilities	18,419	19,911
Net deferred tax asset	$ 47,323	$ 45,729