Derivatives Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2017
Derivative [Line Items]
Convertible Debt 2022 Conversion Feature, Initial Fair Value	$ 28,859,000		$ 28,859,000
Proceeds from Issuance of Warrants			0	$ 14,100,000	$ 12,376,000
Foreign currency forward contracts
Derivative [Line Items]
Notional amount of derivatives, matured during period	50,475,000	$ 43,692,000
Selling, General and Administrative Expenses | Foreign currency forward contracts
Derivative [Line Items]
Gain recognized in income	197,000	52,000	$ 427,000	117,000
Minimum
Derivative [Line Items]
Derivative, percentage of forcasted transactions with currency rate exposure			50.00%
Maximum
Derivative [Line Items]
Derivative, percentage of forcasted transactions with currency rate exposure			90.00%
Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (Loss)	(638,000)	315,000
Cash Flow Hedging [Member] | Sales [Member]
Derivative [Line Items]
Loss on derivative	(209,000)	(166,000)
Cash Flow Hedging [Member] | Cost of Sales [Member]
Derivative [Line Items]
Gain recognized in income		$ 481,000
Loss on derivative	(429,000)
Convertible Subordinated Debt | Convertible Senior Notes at 5.00% February 2021 [Member]
Derivative [Line Items]
Convertible due 2021 - Bond Hedge, Initial Fair Value	27,975,000		$ 27,975,000
Convertible Debt Conversion Feature, Initial Fair Value	(34,480,000)		(34,480,000)
Debt Instrument, Face Amount	$ 150,000,000		$ 150,000,000			$ 150,000,000
Interest rate (as a percent)	5.00%		5.00%
Convertible Subordinated Debt | Convertible Senior Notes at 4.50% February 2022 [Domain]
Derivative [Line Items]
Proceeds from Issuance of Warrants			$ 14,100,000	$ 14,100,000
Convertible due 2022 - Bond Hedge, Fair Value at Issuance	$ 24,780,000		24,780,000
Debt Instrument, Face Amount	$ 120,000,000		$ 120,000,000			$ 120,000,000
Interest rate (as a percent)	4.50%		4.50%