Notes Payable - Net Carrying Amount (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	May 31, 2018
Net carrying amount of the equity component of the Notes
Issuance costs	$ (953,000)
Convertible senior notes, 0.25%, due 2023
Net carrying amount of the liability component of the Notes
Aggregate principal amount	549,999,000	$ 550,000,000.0
Unamortized discount	(84,647,000)	(106,484,000)
Unamortized issuance costs	(7,162,000)	(9,020,000)	$ (10,200,000)
Net carrying amount	458,190,000	434,496,000
Net carrying amount of the equity component of the Notes
Proceeds allocated to the conversion options (debt discount)	119,435,000	119,435,000
Issuance costs	(2,819,000)	(2,819,000)
Net carrying amount	$ 116,616,000	$ 116,616,000