Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Jan. 31, 2013
Policies
Net (loss) income	$ (37,907)	$ (26,017)	$ (23,156)	$ (8,685)	$ (22,960)	$ (17,630)	$ (19,681)	$ (10,004)	$ (95,765)	$ (70,275)	$ (5,152)
Accumulated deficit	345,848				250,083				345,848	250,083
Cash and cash equivalents	15,256				28,947				15,256	28,947	$ 87,176		$ 30,496
Short-term investments	130,860				201,586				130,860	201,586
Outstanding debt with accrued interest	49,624				42,090				49,624	42,090
Restricted cash
Restricted cash, current portion	1,092				901				1,092	901
Restricted cash, long term portion	$ 342				$ 683				$ 342	$ 683
"First Space" Sublease
Restricted cash
Restricted cash														$ 800
"Second Space" Sublease
Restricted cash
Restricted cash												$ 300