Selected Financial Data - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2017	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Reclassification between research and development and general and administrative expenses	$ 1.3	$ 1.3

Reclassification adjustment between research and development and general and administrative expenses.

No definition available.

Name:	arct_ReclassificationAdjustmentBetweenResearchAndDevelopmentAndGeneralAndAdministrativeExpenses
Namespace Prefix:	arct_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration