Lease Exit Liability - Changes in Lease Exit Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Reduction of research and development expense	$ 1,900
Contract termination
Restructuring Reserve [Roll Forward]
Accrual beginning balance	4,713	$ 8,006
Principal payments and other adjustments (net of sublease receipts)	(2,822)	(3,293)
Accrual ending balance	$ 1,891	$ 4,713