License Agreement with Roche (Details)			3 Months Ended		12 Months Ended
	Jan. 01, 2018 USD ($)	Jun. 10, 2016 USD ($) option indication accounting	Dec. 31, 2018 USD ($)	Sep. 15, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Organization And Basis Of Presentation [Line Items]
Change in transaction price			$ 0		$ 0
Milestones achieved in period					0
Calculated under Revenue Guidance in Effect before Topic 606
Organization And Basis Of Presentation [Line Items]
Transaction price					0
Roche
Organization And Basis Of Presentation [Line Items]
Transaction price	$ 30,000,000
Roche
Organization And Basis Of Presentation [Line Items]
License agreement, up-front fee		$ 7,500,000
License agreement, additional up-front fee		$ 262,500,000
License agreement, number of unrelated Indications | indication		2
License agreement, option periods | option		2
License agreement, buyout amount under first option period		$ 135,000,000
License agreement, period to pay buyout option once exercised		30 days
License agreement, buyout amount under second option period		$ 265,000,000
License agreement, buyout amount under second option period, upon non-issuance of patent rights or licensed product		$ 220,000,000
License agreement, period for termination where sufficient development activities are not performed		12 months
Number of units of accounting | accounting		4
Amount of allocable arrangement consideration		$ 29,900,000
Collaboration revenue					$ 0
Roche | EBI-031 | Minimum
Organization And Basis Of Presentation [Line Items]
License agreement, royalty rate		7.50%
Roche | EBI-031 | Maximum
Organization And Basis Of Presentation [Line Items]
License agreement, royalty rate		15.00%
Roche | IL-6
Organization And Basis Of Presentation [Line Items]
License agreement, royalty rate		50.00%
Roche | License revenue
Organization And Basis Of Presentation [Line Items]
Collaboration revenue						$ 400,000	$ 29,600,000
First Indication | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone		$ 197,500,000
Collaborative arrangement, revenue based on development milestone | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone		72,500,000
Collaborative arrangement, revenue based on development milestone | Roche | EBI-031
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone				$ 22,500,000
Collaborative arrangement, revenue based on regulatory milestone | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone		50,000,000
Collaborative arrangement, revenue based on commercialization milestone | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone		75,000,000
Second Indication | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone		$ 65,000,000