Income Taxes - Components of Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Reserve for receivables and inventories	$ 2,210	$ 2,405
Accrued compensation	929	861
Payables	1,090	886
Non-pension postretirement benefits	1,110	1,561
Net operating loss and credit carryforwards	308	364
Accrued pension benefits and deferred compensation	1,552	1,071
Accrued employee benefits	2,534	3,219
Deferred revenue	1,858	1,504
Other		450
Total gross deferred tax assets	11,591	12,321
Less: valuation allowance	(366)	(373)
Total net deferred tax assets	11,225	11,948
Deferred tax liabilities:
Depreciation	4,679	3,778
Amortization	7,146	8,266
Prepaids	517	482
Other	52
Total deferred tax liabilities	12,394	12,526
Net deferred tax liabilities	$ (1,169)	$ (578)