Schedule of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (71,017)	$ (57,843)	$ (68,438)	$ (56,939)	$ (52,720)	$ (46,186)	$ (43,435)	$ (40,463)	$ (254,237)	$ (182,804)	$ (101,526)
Loss attributable to common stockholders—basic	$ (71,017)	$ (57,843)	$ (68,438)	$ (56,939)	$ (52,720)	$ (46,186)	$ (43,435)	$ (40,463)	(254,237)	(182,804)	(101,526)
Effect of dilutive convertible preferred stock									0	0	0
Loss attributable to common stockholders—diluted									$ (254,237)	$ (182,804)	$ (101,526)
Denominator:
Weighted-average number of common shares used in loss per share— basic and diluted (in shares)	44,602,254	43,999,451	43,410,053	43,185,952	43,122,210	43,092,921	43,042,883	43,012,924	43,804,660	43,067,952	39,643,099
Loss per share—basic and diluted (in dollars per share)	$ (1.59)	$ (1.31)	$ (1.58)	$ (1.32)	$ (1.22)	$ (1.07)	$ (1.01)	$ (0.94)	$ (5.80)	$ (4.24)	$ (2.56)