Notes Payable - Net Carrying Amount (Details) - Convertible senior notes, 0.25%, due 2023 - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	May 31, 2018
Net carrying amount of the liability component of the Notes
Aggregate principal amount	$ 549,998,000		$ 549,999,000	$ 550,000,000.0
Unamortized discount	(78,953,000)		(84,647,000)
Unamortized issuance costs	(6,678,000)		(7,162,000)		$ (10,200,000)
Net carrying amount	464,367,000		458,190,000
Net carrying amount of the equity component of the Notes
Proceeds allocated to the conversion options (debt discount)			119,434,000	$ 119,435,000
Issuance costs	(2,819,000)	$ (2,819,000)
Net carrying amount	$ 116,615,000		$ 116,616,000