Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Pension and other postretirement benefits	$ 21,545	$ 29,065
Other employee benefits	15,273	14,544
Deferred revenue	3,963	3,872
Allowance for management fee returned on cancelled policies	3,330	3,515
Unrealized losses on investments	0	1,083
Other	3,484	2,692
Total deferred tax assets	47,595	54,771
Deferred tax liabilities:
Depreciation	35,204	29,978
Unrealized gains on investments	8,713	7,540
Prepaid expenses	2,458	3,800
Other	1,075	1,112
Total deferred tax liabilities	47,450	42,430
Net deferred tax asset	$ 145	$ 12,341