Debt - Debt Discount, Liability, and Equity Allocations of Transaction Costs (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Aug. 06, 2020	Jul. 07, 2020	Jun. 25, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Debt discount	$ 46,700,000	$ 46,700,000					$ 0
Transaction costs
Amortization of transaction costs		2,000,000.0	$ 0
Convertible Senior Notes Due 2025
Debt Instrument [Line Items]
Debt discount, excluding amortization		39,000,000.0
Transaction costs
Liability	5,500,000	5,500,000
Equity	1,200,000	1,200,000
Total transaction costs	6,700,000	6,700,000
Convertible Senior Notes Due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt discount, excluding amortization	2,500,000	2,700,000
Debt discount						$ 42,600,000
Transaction costs
Total transaction costs						1,400,000
Aggregate principal amount of debt						225,000,000
Over-Allotment Option
Debt Instrument [Line Items]
Debt discount	3,600,000	3,600,000
Transaction costs
Liability	700,000	700,000
Equity	200,000	200,000
Total transaction costs	900,000	900,000
Over-Allotment Option | Convertible Debt
Debt Instrument [Line Items]
Debt discount				$ 3,600,000
Transaction costs
Total transaction costs				$ 200,000	$ 7,600,000
Amortization of transaction costs	300,000	300,000
Aggregate principal amount of debt	20,750,000	20,750,000			$ 20,750,000	33,750,000
2025 Notes | Convertible Debt
Debt Instrument [Line Items]
Debt discount						$ 42,600,000
Transaction costs
Liability	6,200,000	6,200,000
Equity	1,400,000	1,400,000
Total transaction costs	$ 7,600,000	$ 7,600,000