Restructuring Activities (Details) $ in Thousands		3 Months Ended	6 Months Ended	9 Months Ended
	Jan. 26, 2017 USD ($)	Mar. 31, 2017 USD ($) employee	Sep. 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		74
Cash expenditures related to reduction in personnel		$ 4,781
Costs related to reduction in personnel		6,700
Accrual balance for restructuring activity		3,784			$ 0
Accrued restructuring expenses		8,565
Severance and other employee costs | Research and development
Restructuring Cost and Reserve [Line Items]
Costs related to reduction in personnel		4,800
Severance and other employee costs | General and administrative
Restructuring Cost and Reserve [Line Items]
Costs related to reduction in personnel		1,900
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			$ 13,200	$ 13,200
Costs expected to result in future cash expenditures				$ 12,200
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			45
Expected costs			$ 11,100	$ 11,100
Subsequent Event | Forecast | Lease termination costs
Restructuring Cost and Reserve [Line Items]
Expected costs			$ 2,100	$ 2,100