Debt and Credit Facilities (Schedule of Convertible Debt Interest Expense) (Details) - Convertible Senior Notes due 2022 - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Debt Instrument [Line Items]
Coupon interest expense	$ 632	$ 1,265
Amortization of debt discount - equity component	335	662
Amortization of debt discount - initial purchaser	101	198
Amortization of transaction costs	16	31
Total interest expense	$ 1,084	$ 2,156