Convertible Senior Notes and Capped Call Transactions (Carrying Amount and Interest Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 31, 2019
Carrying Amount of Equity Component:
Proceeds allocated to the conversion options (debt discount)						$ 52,900
Interest Expense:
Amortization of issuance costs			$ 600	$ 376
Amortization of debt discount			4,777	2,990
Convertible Debt | Convertible Senior Notes
Carrying Amount of Liability Component:
Principal	$ 230,000		230,000		$ 230,000
Unamortized discount	(40,518)		(40,518)		(45,295)
Unamortized issuance costs	(5,093)		(5,093)		(5,693)
Net carrying amount	184,389		184,389		$ 179,012
Carrying Amount of Equity Component:
Proceeds allocated to the conversion options (debt discount)	52,900		52,900
Issuance costs	(1,986)		(1,986)
Net carrying amount	50,914		50,914
Interest Expense:
Contractual interest expense	431	$ 431	863	575
Amortization of issuance costs	303	260	600	376
Amortization of debt discount	2,409	2,263	4,776	2,990
Total interest expense	$ 3,143	$ 2,954	$ 6,239	$ 3,941