Debt - Interest Expense - Convertible Senior Notes (Details) - Convertible Notes - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash Interest Expense
Coupon interest expense	$ 9,315	$ 9,315
Non-Cash Interest Expense
Amortization of debt discount	11,516	10,686
Amortization of transaction costs	1,160	1,076
Total Interest Expense	21,991	$ 21,077
Estimated fair value	$ 330,300
Convertible Debt, Fair Value by Fair Value Hierarchy Level	us-gaap:FairValueInputsLevel2Member