Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Allowance for loan losses	$ 44,486	$ 42,882
Depreciation	0	19,111
Employee benefits	28,263	13,301
Investment securities, including other-than-temporary impairment losses	0	13,222
Net operating loss carryforwards	19,768	21,570
Purchase accounting	41,857	33,629
Other	19,904	22,104
Total deferred tax assets	154,278	165,819
Deferred tax liabilities:
Pension plans	19,686	18,786
Depreciation	4,527	0
Investment securities, including other-than-temporary impairment losses	2,319	0
Other investments	7,731	17,758
Core deposit intangibles	16,620	14,223
Other	13,665	8,858
Total deferred tax liabilities	64,548	59,625
Valuation Allowance	916	733
Net deferred tax asset (included in other assets)	$ 88,814	$ 105,461