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Fair Value Measurements - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2019	Nov. 30, 2017	Mar. 31, 2020	Dec. 31, 2019
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Asset impairment charges			$ 0
Fair value of assets transferred from level 1 to level 2			0
Fair value of assets transferred from level 2 to level 1			0
Fair value of liabilities transferred from level 1 to level 2			0
Fair value of liabilities transferred from level 2 to level 1			0
Carrying value of convertible senior notes	$ 430,282,000		435,718,000	$ 430,282,000
0.125% Convertible Senior Notes Due 2024
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of convertible senior notes	450,400,000		518,800,000	450,400,000
Carrying value of convertible senior notes	$ 351,100,000		$ 355,500,000	$ 351,100,000
Debt instrument, interest rate	0.125%		0.125%	0.125%
Debt instrument, maturity date	Dec. 15, 2024		Dec. 15, 2024
1.50% Convertible Senior Notes Due 2022
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of convertible senior notes	$ 215,800,000		$ 306,400,000	$ 215,800,000
Carrying value of convertible senior notes	79,200,000		$ 80,300,000	79,200,000
Debt instrument, interest rate		1.50%	1.50%
Debt instrument, maturity date		Nov. 01, 2022	Nov. 01, 2022
Fair Value Measurements, Nonrecurring
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Asset impairment charges			$ 0	0
Fair Value Measurements, Recurring
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial assets	511,436,000		473,254,000	511,436,000
Financial liabilities			2,530,000
Fair Value Measurements, Recurring \| Level 2 Securities
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial assets	0		0	0
Financial liabilities	$ 0		$ 0	$ 0