Net Loss Per Share - Schedule of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
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
Numerator:
Net loss	$ (24,715)	$ (30,044)	$ (35,474)	$ (42,760)	$ (41,098)	$ (49,802)	$ (68,882)	$ (61,555)	$ (132,993)	$ (221,337)	$ (254,237)
LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS—BASIC	$ (24,715)	$ (30,044)	$ (35,474)	$ (42,760)	$ (41,098)	$ (49,802)	$ (68,882)	$ (61,555)	(132,993)	(221,337)	(254,237)
LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS—DILUTED									$ (132,993)	$ (221,337)	$ (254,237)
Denominator:
Weighted-average number of common shares used in loss per share— basic and diluted (in shares)	46,176,145	46,141,217	46,109,193	45,671,502	45,549,972	45,498,909	45,430,678	44,937,776	46,026,217	45,356,263	43,804,660
Loss per share—basic and diluted (in dollars per share)	$ (0.54)	$ (0.65)	$ (0.77)	$ (0.94)	$ (0.90)	$ (1.09)	$ (1.52)	$ (1.37)	$ (2.89)	$ (4.88)	$ (5.80)

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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