Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Pension and other postretirement benefits	$ 29,065	$ 25,720
Other employee benefits	14,544	11,835
Deferred revenue	3,872	3,755
Allowance for management fee returned on cancelled policies	3,515	3,421
Unrealized losses on investments	1,083	0
Other	2,692	1,663
Total deferred tax assets	54,771	46,394
Deferred tax liabilities:
Depreciation	29,978	19,454
Unrealized gains on investments	7,540	4,181
Prepaid expenses	3,800	4,890
Commissions	884	545
Other	228	138
Total deferred tax liabilities	42,430	29,208
Net deferred tax asset	$ 12,341	$ 17,186