Restructuring Activities (Details) $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended
	Jan. 26, 2017 USD ($)	Jan. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) employee	Jun. 30, 2017 USD ($) employee	Jun. 30, 2017 USD ($) employee	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Severance and other employee costs
Restructuring Cost and Reserve [Line Items]
Number of employee positions eliminated in reduction of personnel | employee				98	98
Cash expenditures related to reduction in personnel					$ 8,039
Costs related to reduction in personnel				$ 1,800	10,500
Accrual balance for restructuring activity				1,967	1,967		$ 0
Accrued restructuring expenses					10,006
Severance and other employee costs | Research and development
Restructuring Cost and Reserve [Line Items]
Costs related to reduction in personnel				1,100	5,900
Severance and other employee costs | General and administrative
Restructuring Cost and Reserve [Line Items]
Costs related to reduction in personnel				$ 700	$ 4,600
Lease termination costs | One Penn Plaza LLC
Restructuring Cost and Reserve [Line Items]
Accrued restructuring expenses	$ 900
Lease termination costs | Otsuka America Pharmaceutical, Inc.
Restructuring Cost and Reserve [Line Items]
Accrued restructuring expenses	$ 1,200
Lease termination costs | General and administrative
Restructuring Cost and Reserve [Line Items]
Accrued restructuring expenses		$ 2,100
Subsequent Event | Forecast
Restructuring Cost and Reserve [Line Items]
Expected costs			$ 13,400			$ 13,400
Costs expected to result in future cash expenditures						$ 12,500
Subsequent Event | Forecast | Severance and other employee costs
Restructuring Cost and Reserve [Line Items]
Percent of personnel expected to be effected by reduction in workforce			80.00%			80.00%
Number of employee positions remaining expected to be affected by reduction in personnel | employee			20
Expected costs			$ 11,300			$ 11,300
Subsequent Event | Forecast | Lease termination costs
Restructuring Cost and Reserve [Line Items]
Expected costs			$ 2,100			$ 2,100