Acquisition of Confluence - Summary of fair valuel consideration (Details) - USD ($) $ in Thousands		9 Months Ended
	Aug. 03, 2017	Sep. 30, 2017
Fair value of consideration
Cash consideration paid		$ 9,647
Confluence
Business Acquisition [Line Items]
Acquisition of outstanding common shares (as a percent)	100.00%
Additional contingent consideration based on milestones, maximum, per Agreement	$ 80,000
Additional contingent consideration based on milestones, portion payable in shares	$ 2,500
Royalty term	10 years
Fair value of consideration
Cash consideration paid	$ 10,269
Aclaris common stock issued	9,675
Contingent consideration	4,378
Total fair value of consideration to Confluence equity holders	$ 24,322