Convertible Senior Notes and Capped Call Transactions (Carrying Amount and Interest Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Carrying Amount of Equity Component:
Proceeds allocated to the conversion options (debt discount)	$ 52,900
Interest Expense:
Amortization of issuance costs	956	$ 0	$ 0
Amortization of debt discount	7,605	$ 0	$ 0
Convertible Debt | Convertible Senior Notes
Carrying Amount of Liability Component:
Principal	230,000
Unamortized discount	(45,295)
Unamortized issuance costs	(5,693)
Net carrying amount	179,012
Carrying Amount of Equity Component:
Proceeds allocated to the conversion options (debt discount)	52,900
Issuance costs	(1,986)
Net carrying amount	50,914
Interest Expense:
Contractual interest expense	1,438
Amortization of issuance costs	956
Amortization of debt discount	7,605
Total interest expense	$ 9,999