Liabilities - Convertible Senior Notes - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Non-Cash Interest Expense
Total Interest Expense	$ 6,100	$ 5,467	$ 11,750	$ 10,877
2023 Notes
Cash Interest Expense
Coupon interest expense	2,070	2,329	4,399	4,658
Non-Cash Interest Expense
Amortization of debt discount	2,729	2,851	5,746	5,650
Amortization of transaction costs	$ 275	$ 287	$ 579	$ 569
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member		us-gaap:FairValueInputsLevel2Member
Estimated fair value	$ 235,400		$ 235,400
2027 Notes
Cash Interest Expense
Coupon interest expense	573		573
Non-Cash Interest Expense
Amortization of debt discount	425		425
Amortization of transaction costs	$ 28		$ 28
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member		us-gaap:FairValueInputsLevel2Member
Estimated fair value	$ 141,500		$ 141,500