Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Significant components of deferred tax assets and liabilities
Post-retirement benefits	$ 1,160	$ 1,798
Inventory reserves	1,128	1,834
Loss carry-forwards	5,401	7,279
Credit carry-forwards	10,793	22,743
Nondeductible accruals	7,062	11,629
Research expenditures	20,002	31,380
Stock compensation	1,803	2,681
Foreign exchange loss	1,373	1,780
Other	220	648
Gross deferred tax assets	48,942	81,772
Depreciation and amortization	9,819	9,960
Pensions	12,387	16,024
Subsidiaries' unremitted earnings	1,662	1,292
Gross deferred tax liabilities	23,868	27,276
Net deferred tax assets	25,074	54,496
Deferred tax asset valuation allowance	(8,182)	(11,024)
Total net deferred tax assets	$ 16,892	$ 43,472