Summary of Significant Accounting Policies - Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (15,676)	$ (15,789)	$ (18,413)	$ (17,820)	$ (10,066)	$ (16,721)	$ (15,461)	$ (14,762)	$ (67,698)	$ (57,010)	$ (10,380)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue									(33,558)	(11,442)	28,029
Cash, cash equivalents and restricted cash at beginning of period				224,857				136,057	224,857	136,057	112,555
Cash, cash equivalents and restricted cash at end of period	$ 248,265				224,857				248,265	224,857	136,057
As Originally Reported | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss										(45,952)	(13,152)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue										(22,500)	30,801
Cash, cash equivalents and restricted cash at beginning of period				224,571				135,797	224,571	135,797	112,265
Cash, cash equivalents and restricted cash at end of period					224,571					224,571	135,797
Effect of Change | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss										(11,058)	2,772
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue										11,058	(2,772)
Cash, cash equivalents and restricted cash at beginning of period				$ 286				$ 260	$ 286	260	290
Cash, cash equivalents and restricted cash at end of period					$ 286					$ 286	$ 260