Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Pension and other postretirement benefits	$ 25,720	$ 20,124
Other employee benefits	11,835	12,237
Deferred revenue	3,755	3,524
Allowance for management fee returned on cancelled policies	3,421	3,292
Unrealized losses on investments	0	2,030
Other	1,663	1,246
Total deferred tax assets	46,394	42,453
Deferred tax liabilities:
Depreciation	19,454	13,015
Prepaid expenses	4,890	1,376
Limited partnerships	2,632	2,534
Unrealized gains on investments	1,549	0
Commissions	545	1,270
Other	138	157
Total deferred tax liabilities	29,208	18,352
Net deferred tax asset	$ 17,186	$ 24,101