Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended	3 Months Ended
	Apr. 13, 2020	Oct. 17, 2019	Apr. 26, 2019	Jan. 01, 2019	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Oct. 31, 2019	Apr. 30, 2019	Apr. 23, 2019	Dec. 31, 2018	Aug. 31, 2017	Aug. 14, 2017	Aug. 08, 2017
Policies
Net (loss) income						$ (35,372)	$ (34,091)
Cumulative-effect adjustment upon adoption				$ 400		403
Accumulated deficit						734,946	699,200	$ 699,171
Cash and cash equivalents						74,144		61,926
Marketable securities						331,681		379,065
Short-term Credit Line, outstanding balance						50,087		50,123
Long-term Term Loan, carrying amount						$ 72,881		$ 73,656
Common stock, shares issued						78,652,000		78,005,000
Payment of offering expenses		$ 400	$ 600
Proceeds from issuance of common stock		$ 216,200	$ 108,100
Aggregate principal amount						$ 75,000		$ 75,000
Debt principal balance						75,000		$ 75,000
2017 Term Loan
Policies
Long-term Term Loan, carrying amount						72,900
Remaining borrowing capacity										$ 50,000		$ 50,000	$ 50,000		$ 25,000
Common stock, shares issued										25,000			300,000	300,000
Stock issued (in dollars per share)														$ 8.16
Aggregate principal amount														$ 75,000
Debt principal balance														$ 75,000
Subsequent Event | 2.25% Convertible Senior Notes
Policies
Aggregate principal amount	$ 287,500				$ 287,500
Interest rate (as a percent)	2.25%				2.25%
Common stock that may be issued upon conversion of notes	9,634,700				9,634,700
Net proceeds	$ 278,900				$ 278,900
Debt principal balance	$ 287,500				287,500
Subsequent Event | 2017 Term Loan
Policies
Aggregate principal amount					75,000
Repayment of Term Loan					79,200
Debt principal balance					75,000
Debt extinguishment costs					$ 4,200
Equity offering
Policies
Common stock, shares issued									6,571,428		6,052,631
Stock issued (in dollars per share)		$ 35									$ 19
Accumulated Deficit
Policies
Net (loss) income						(35,372)	(34,091)
Cumulative-effect adjustment upon adoption						$ 403	$ (185)