Convertible Senior Notes - Convertible Notes and Options (Details)	1 Months Ended	2 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2018 USD ($) day $ / shares	Jun. 30, 2018 USD ($)	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Net carrying amount			$ 122,511,000		$ 122,511,000		$ 117,178,000
Total interest expense			2,368,000	$ 2,233,000	6,950,000	$ 6,524,000
Convertible Debt | Convertible Senior Notes at 1.50%
Debt Instrument [Line Items]
Aggregate principal amount	$ 143,750,000		143,750,000		143,750,000		143,750,000
Stated interest rate	1.50%
Proceeds from issuance of convertible senior notes, net of issuance costs	$ 138,900,000
Unamortized debt discount			(19,087,000)		(19,087,000)		(23,880,000)
Unamortized issuance costs			(2,152,000)		(2,152,000)		(2,692,000)
Net carrying amount			122,511,000		122,511,000		117,178,000
Convertible Debt | Convertible Senior Notes At 1.50%, Option Portion
Debt Instrument [Line Items]
Aggregate principal amount	$ 18,750,000
Stated interest rate	1.50%
Conversion ratio	0.0310073
Convertible debt, conversion price (in dollars per share) | $ / shares	$ 32.25
Denominator in calculation of trading price	$ 1,000
Conditional cash repurchase, percent of principal	100.00%
Carrying amount of the equity component	$ 33,400,000		33,350,000		33,350,000		33,350,000
Debt discount effective interest rate		7.60%
Amortization of debt issuance costs		$ 4,900,000	185,000	172,000	540,000	498,000
Amortization of debt issuance costs, liability component		3,800,000
Amortization of debt issuance costs, equity component		$ 1,100,000
Issuance costs			(1,136,000)		(1,136,000)		(1,136,000)
Net carrying amount			32,214,000		32,214,000		$ 32,214,000
Estimated fair value of the Notes			161,700,000		161,700,000
Denominator in closing price calculation			$ 100		$ 100
If-converted value, share price (in dollars per share) | $ / shares			$ 29.08		$ 29.08
If-converted value of the Notes					$ 129,600,000
Contractual interest expense			$ 539,000	539,000	1,617,000	1,617,000
Amortization of debt discount			1,644,000	1,521,000	4,793,000	4,414,000
Total interest expense			$ 2,368,000	$ 2,232,000	$ 6,950,000	$ 6,529,000
Convertible Debt | Convertible Senior Notes At 1.50%, Option Portion | Maximum
Debt Instrument [Line Items]
Convertible debt, threshold trading days | day	20
Convertible debt, threshold consecutive trading days | day	30
Percent of conversion price triggering conversion feature	130.00%
Convertible Debt | Convertible Senior Notes At 1.50%, Option Portion | Minimum
Debt Instrument [Line Items]
Convertible debt, threshold trading days | day	5
Convertible debt, threshold consecutive trading days | day	10
Percent of conversion price triggering conversion feature	98.00%