Description of Business - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	[1]
Description Of Business [Line Items]
Accumulated deficit	$ (1,148,448)	$ (1,140,083)
Cash, cash equivalents and marketable securities	$ 46,600
Servier [Member]
Description Of Business [Line Items]
Collaboration agreement termination date	Mar. 26, 2016

[1] The condensed consolidated balance sheet as of December 31, 2015 has been derived from the audited consolidated financial statements as of that date included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015.