Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Non-Cash Interest Expense
Amortization of debt discount/transaction costs	$ 13,792	$ 12,676	$ 11,762
Total Interest Expense	24,640	21,991	21,077
2023 Notes
Cash Interest Expense
Coupon interest expense	7,390	9,315	9,315
Non-Cash Interest Expense
Amortization of debt discount/transaction costs	10,887	$ 12,676	$ 11,762
Estimated fair value	$ 125,400
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member
2025 Notes
Cash Interest Expense
Coupon interest expense	$ 554
Non-Cash Interest Expense
Amortization of debt discount/transaction costs	546
Estimated fair value	$ 140,200
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member
2027 Notes
Cash Interest Expense
Coupon interest expense	$ 2,904
Non-Cash Interest Expense
Amortization of debt discount/transaction costs	2,359
Estimated fair value	$ 180,900
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member