Liabilities - Convertible Senior Notes - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Non-Cash Interest Expense
Total Interest Expense	$ 6,409	$ 5,528	$ 18,087	$ 16,404
2023 Notes
Cash Interest Expense
Coupon interest expense	1,733	2,329	6,060	6,986
Non-Cash Interest Expense
Amortization of debt discount	2,328	2,906	8,074	8,556
Amortization of transaction costs	$ 235	$ 293	$ 813	$ 862
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member		us-gaap:FairValueInputsLevel2Member
Estimated fair value	$ 235,900		$ 235,900
2027 Notes
Cash Interest Expense
Coupon interest expense	1,172		1,745
Non-Cash Interest Expense
Amortization of debt discount	884		1,310
Amortization of transaction costs	$ 57		$ 85
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member		us-gaap:FairValueInputsLevel2Member
Estimated fair value	$ 134,800		$ 134,800