Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Allowance for loan losses	$ 42,882	$ 34,885
Depreciation	19,111	8,336
Employee benefits	13,301	10,596
Investment securities, including other-than-temporary impairment losses	13,222	5,021
Net operating loss carryforwards	21,570	30,658
Purchase accounting	33,629	18,819
Capital loss carryforward	830	0
Other	21,274	21,930
Total deferred tax assets	165,819	130,245
Deferred tax liabilities:
Pension plans	18,786	18,912
Other investments	17,758	13,234
Deferred income	0	37,952
Core deposit intangibles	14,223	5,182
Other	8,858	7,469
Total deferred tax liabilities	59,625	82,749
Valuation Allowance	733	0
Net deferred tax asset (included in other assets)	$ 105,461	$ 47,496