Summary of Significant Accounting Policies - Condensed Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Condensed Consolidated Balance Sheet Data (in thousands):
Prepaid expenses and other current assets	$ 4,605		$ 4,605			$ 5,290	$ 4,574
Other non-current assets	11,490		11,490			2,030	910
Total Assets	130,840		130,840			143,177	141,341
Deferred revenues, current portion	28,406		28,406				35,612
Deferred revenue, net of current portion	58,110		58,110			56,780	53,303
Total Liabilities	111,355		111,355			104,883	101,406
Accumulated deficit	(170,813)		(170,813)			(148,707)	(147,066)
Total stockholders' equity	19,485	$ 55,317	19,485	$ 55,317	$ 30,162	38,294	39,935	$ 50,692	$ 11,522
Total liabilities and stockholders' equity	130,840		130,840			143,177	$ 141,341
Condensed Consolidated Statement of Operations Data (in thousands):
Revenue	5,332	11,691	13,877	15,843
General and administrative expenses	4,189	4,779	9,121	9,131
Loss from operations	(12,230)	(2,243)	(22,913)	(10,379)
Loss before income taxes	(11,758)	(351)	(22,106)	(9,065)
Net loss	(11,758)	(203)	(22,106)	(8,917)
Comprehensive loss	(12,409)	$ 3,196	(21,831)	$ (6,796)
Adjusted Balance, ASC 605
Condensed Consolidated Balance Sheet Data (in thousands):
Prepaid expenses and other current assets	4,001		4,001
Other non-current assets	10,370		10,370
Total Assets	129,116		129,116
Deferred revenues, current portion	37,095		37,095
Deferred revenue, net of current portion	46,466		46,466
Total Liabilities	108,400		108,400
Accumulated deficit	(169,582)		(169,582)
Total stockholders' equity	20,716		20,716
Total liabilities and stockholders' equity	129,116		129,116
Condensed Consolidated Statement of Operations Data (in thousands):
Revenue	4,133		13,355
General and administrative expenses	4,150		9,009
Loss from operations	(13,468)		(23,547)
Loss before income taxes	(12,996)		(22,740)
Net loss	(12,996)		(22,740)
Comprehensive loss	(13,647)		(22,465)
Accounting Standards Update 2014-09
Condensed Consolidated Balance Sheet Data (in thousands):
Prepaid expenses and other current assets						716
Other non-current assets						1,120
Total Assets						1,836
Deferred revenue, net of current portion						3,477
Total Liabilities						3,477
Accumulated deficit						(1,641)
Total stockholders' equity						(1,641)
Total liabilities and stockholders' equity						$ 1,836
Accounting Standards Update 2014-09 | Adjustments
Condensed Consolidated Balance Sheet Data (in thousands):
Prepaid expenses and other current assets	(604)		(604)
Other non-current assets	(1,120)		(1,120)
Total Assets	(1,724)		(1,724)
Deferred revenues, current portion	8,689		8,689
Deferred revenue, net of current portion	(11,644)		(11,644)
Total Liabilities	(2,955)		(2,955)
Accumulated deficit	1,231		1,231
Total stockholders' equity	1,231		1,231
Total liabilities and stockholders' equity	(1,724)		(1,724)
Condensed Consolidated Statement of Operations Data (in thousands):
Revenue	(1,199)		(522)
General and administrative expenses	(39)		(112)
Loss from operations	(1,238)		(634)
Loss before income taxes	(1,238)		(634)
Net loss	(1,238)		(634)
Comprehensive loss	$ (1,238)		$ (634)