Convertible Senior Notes and Capped Call Transactions (Carrying Amount and Interest Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Carrying Amount of Equity Component:
Proceeds allocated to the conversion options (debt discount)	$ 52,900
Interest Expense:
Amortization of debt issuance costs	116	$ 0
Amortization of debt discount	727	$ 0
Convertible Debt | Convertible Senior Notes
Carrying Amount of Liability Component:
Principal	230,000
Unamortized discount	(52,173)
Unamortized issuance costs	(6,533)
Net carrying amount	171,294
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	144
Amortization of debt issuance costs	116
Amortization of debt discount	727
Total interest expense	$ 987