Debt and Credit Facilities Debt and Credit Facilities (Schedule of Convertible Debt Interest Expense) (Details) - Convertible Senior Notes due 2022 $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Coupon interest expense	$ 633
Amortization of debt discount - equity component	327
Amortization of debt discount - initial purchaser	97
Amortization of transaction costs	15
Total interest expense	$ 1,072