License Agreement with Roche (Details) $ in Thousands		3 Months Ended		12 Months Ended
	Jun. 10, 2016 USD ($) option accounting	Sep. 30, 2016 USD ($)	Sep. 15, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Organization And Basis Of Presentation [Line Items]
Collaboration revenue				$ 0	$ 406	$ 490
Roche
Organization And Basis Of Presentation [Line Items]
License agreement, up-front fee	$ 7,500
License agreement, additional up-front fee	$ 262,500
License agreement, number of unrelated Indications	2
License agreement, option periods | option	2
License agreement, buyout amount under first option period	$ 135,000
License agreement, period to pay buyout option once exercised	30 days
License agreement, buyout amount under second option period	$ 265,000
License agreement, buyout amount under second option period, upon non-issuance of patent rights or licensed product	$ 220,000
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
Collaboration revenue		$ 29,000		$ 400
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%
First Indication | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone	$ 197,500
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
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
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
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
Second Indication | Roche
Organization And Basis Of Presentation [Line Items]
License agreement, amount payable upon achievement of specified milestone	$ 65,000