Convertible Senior Notes and Capped Call Transactions - Carrying Amount and Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest Expense:
Amortization of issuance costs	$ 1,340	$ 956	$ 0
Amortization of debt discount	11,564	7,605	$ 0
Convertible Debt | Convertible Senior Notes
Carrying Amount of Liability Component:
Principal	747,500	230,000
Unamortized discount	(196,269)	(45,295)
Unamortized issuance costs	(12,799)	(5,693)
Net carrying amount	538,432	179,012
Carrying Amount of Equity Component:
Proceeds allocated to the conversion options (debt discount)	215,434	52,900
Issuance costs	(5,783)	(1,986)
Net carrying amount	209,651	50,914
Interest Expense:
Contractual interest expense	1,725	1,438
Amortization of issuance costs	1,340	956
Amortization of debt discount	11,564	7,605
Total interest expense	$ 14,629	$ 9,999