Restructuring Charge (Details)	1 Months Ended	3 Months Ended
	Feb. 28, 2018 employee	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 ft²	Feb. 29, 2016 ft²
Restructuring
Stock compensation expense (reduction)		$ 3,700,000	$ 2,600,000
Workforce reduction
Restructuring
Approximate positions to be eliminated | employee	30
Approximate current positions to be eliminated | employee	20
One-time charge for severance		1,200,000
Benefit plan severance cost		384,000
930 Winter Street, Walham, MA
Restructuring
Area of space leased | ft²					10,281
930 Winter Street, Walham, MA | Lease
Restructuring
Area of space leased | ft²				10,281
Leasehold impairment charge		0	$ 386,000
Stock options | Workforce reduction
Restructuring
Stock compensation expense (reduction)		$ 157,000