Debt and Credit Facilities Debt and Credit Facilities (Schedule of Carrying Value of the Notes) (Details) - Convertible Senior Notes due 2022 - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Oct. 11, 2017	Sep. 25, 2017
Debt Instrument [Line Items]
Principal amount	$ 46,000	$ 46,000	$ 6,000	$ 40,000
Unamortized debt discount - equity component	(7,817)	(8,144)
Unamortized debt discount - initial purchaser	(2,334)	(2,431)
Unamortized transaction costs	(368)	(383)
Unamortized transaction costs	$ 35,481	$ 35,042