Restructuring Charge (Details)	1 Months Ended	3 Months Ended								6 Months Ended	9 Months Ended			12 Months Ended
	Sep. 30, 2016 USD ($) ft² item	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) ft²	Sep. 30, 2016 USD ($) ft²	Dec. 31, 2016 USD ($) ft² shares	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2016 USD ($) ft²	Dec. 31, 2018 USD ($) employee shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) ft²	Jun. 30, 2016 USD ($) shares
Restructuring
Stock compensation expense (reduction)										$ 8,100,000				$ 16,400,000	$ 11,100,000		$ 21,900,000
Leasehold impairment charge										970,000					180,000
Restructuring Reserve
Initial charge related to employee benefits, Beginning Balance				$ 1,189,000							$ 1,189,000
Additional charges during the year		$ 406,000	$ 870,000	$ 686,000	$ 1,731,000	$ 393,000	$ 386,000	$ 301,000	$ 4,130,000	4,431,000	2,347,000			3,693,000	779,000	$ 4,431,000
Payments during the period											(2,695,000)
Balance at the End of the year		$ 841,000			1,189,000						$ 841,000			$ 841,000
Workforce reduction
Restructuring
Approximate positions to be eliminated (as a percent)	17.00%
Approximate positions to be eliminated	65													22
Approximate current positions to be eliminated | item	60
One-time charge for severance					1,200,000					2,800,000
Benefit plan severance cost										593,000		$ 2,300,000
Restructuring Reserve
Additional charges during the year										$ 4,400,000
930 Winter Street, Walham, MA
Restructuring
Area of space leased | ft²								10,281		10,281						10,281
930 Winter Street, Walham, MA | Lease
Restructuring
Area of space leased | ft²	10,281								10,281				10,281
Estimated period to sub-lease													9 months
Leasehold impairment charge													$ 970,000		$ 779,000
Leasehold improvement cost	$ 193,000								$ 193,000				$ 193,000
Stock options
Restructuring
Forfeited (in shares) | shares										1,670,000				1,178,000	3,106,000		661,000
Granted (in shares) | shares										3,536,000				5,513,000	1,589,000		3,340,000
Stock options | Workforce reduction
Restructuring
Forfeited (in shares) | shares										762,000
Stock compensation expense (reduction)					$ 157,000					$ (837,000)