Income Taxes - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses	$ 96,508	$ 44,486
Employee benefits	20,888	28,263
Net operating loss carryforwards	17,814	19,768
Purchase accounting	10,354	41,857
Other	24,677	19,904
Total deferred tax assets	170,241	154,278
Deferred tax liabilities:
Pension plans	22,705	19,686
Depreciation	3,829	4,527
Investment securities	12,690	2,319
Other investments	9,584	7,731
Core deposit intangibles	12,960	16,620
Other	12,226	13,665
Total deferred tax liabilities	73,994	64,548
Valuation Allowance	916	916
Net deferred tax asset (included in other assets)	$ 95,331	$ 88,814