Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Net loss	$ (23,852)	$ (10,217)	$ (55,577)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue	(92)	(463)	(1,890)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347	53,347
Cash, cash equivalents and restricted cash, end of period	42,113	47,985	39,992
ASC 606 | As Originally Reported Under ASC 605
Net loss		(10,523)	(57,215)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		(157)	(252)
Cash, cash equivalents and restricted cash, beginning of period	$ 39,992	53,347	53,347
Cash, cash equivalents and restricted cash, end of period		47,985	39,992
ASC 606 | Effect of Change
Net loss		306	1,638
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		$ (306)	$ (1,638)