Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Allowance for loan losses	$ 44,382	$ 42,574
Depreciation	15,661	15,082
Employee benefits	16,104	17,893
Investment securities, including other-than-temporary impairment losses	18,697	15,972
Net operating loss carryforwards	57,722	39,462
Purchase accounting	40,585	31,327
Other	21,310	18,138
Total deferred tax assets	214,461	180,448
Deferred tax liabilities:
Pension plans	18,861	13,081
Other investments	15,720	14,662
Other	21,449	21,127
Total deferred tax liabilities	56,030	48,870
Net deferred tax asset (included in other assets)	$ 158,431	$ 131,578