Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended			3 Months Ended								12 Months Ended
	Oct. 17, 2019	Apr. 26, 2019	Oct. 31, 2019	Apr. 30, 2019	Jul. 31, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 23, 2019	Aug. 31, 2017	Aug. 14, 2017	Aug. 08, 2017	Dec. 31, 2016
Policies
Net (loss) income						$ (35,183)	$ (23,137)	$ (32,416)	$ (34,091)	$ (31,841)	$ (29,616)	$ (33,824)	$ (32,873)	$ (124,827)	$ (128,154)	$ (137,628)
Accumulated deficit						699,171				574,529				699,171	574,529
Cash, cash equivalents and restricted cash						61,981				51,004				61,981	51,004	13,021					$ 16,690
Marketable securities						379,065				107,461				379,065	107,461
Short-term Credit Line, outstanding balance						50,123				50,153				50,123	50,153
Long-term Term Loan, carrying amount						$ 73,656				$ 73,357				$ 73,656	$ 73,357
Common stock, shares issued	5,714,286	789,473	5,714,286	5,263,158	4,500,000	78,004,669				62,083,000				78,004,669	62,083,000		5,263,158
Stock issued (in dollars per share)	$ 35		$ 35	$ 19	$ 20												$ 19
Payment of offering expenses	$ 400	$ 600	$ 400	$ 600	$ 500
Proceeds from issuance of common stock	$ 216,200	$ 108,100	$ 216,200	108,100	$ 97,300
2017 Term Loan
Policies
Long-term Term Loan, carrying amount						$ 73,656				$ 73,357				$ 73,656	$ 73,357
Remaining borrowing capacity				$ 50,000					$ 50,000	$ 25,000					25,000			$ 50,000		$ 25,000
Common stock, shares issued				25,000														300,000	300,000
Stock issued (in dollars per share)																				$ 8.16
Option for additional shares
Policies
Common stock, shares issued	857,142	789,473	857,142		675,000
Accumulated Deficit
Policies
Net (loss) income														(124,827)	$ (128,154)	$ (137,628)
Cumulative-effect adjustment upon adoption of ASU 2018-07														$ (185)