Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Significant components of deferred tax assets and liabilities
Post-retirement benefits	$ 1,259	$ 1,100
Inventory reserves	477	708
Loss carry-forwards	5,128	4,724
Credit carry-forwards	17,401	15,964
Accrued expenses	5,693	4,932
Research expenditures	18,893	17,953
Operating lease liabilities	6,012	6,211
Stock compensation	1,969	2,232
Foreign exchange loss	2,166	1,986
Other	872	230
Gross deferred tax assets	59,870	56,040
Depreciation and amortization	13,004	12,453
Pensions	12,557	13,552
Operating lease assets	5,703	5,963
Subsidiaries' unremitted earnings	3,046	1,903
Gross deferred tax liabilities	34,310	33,871
Net deferred tax assets	25,560	22,169
Deferred tax asset valuation allowance	(8,320)	(8,011)
Total net deferred tax assets	$ 17,240	$ 14,158