Accounting Policies - Foreign Currency and Research and Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Currency translation adjustments	$ (35,928)	$ (54,145)
Derivative hedging activity	3,336	1,004
Pension activity	(244)	(359)
Accumulated other comprehensive loss	(32,836)	(53,500)
Foreign currency transaction gain (loss)	(3,300)	(900)	$ (100)
Operating expenses
Financing Receivable, Impaired [Line Items]
Research and development costs	$ 4,200	$ 4,700	$ 2,600