Commitments and Contingencies - Construction Commitments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Construction Commitment [Line Items]
Committed construction cost (in thousands)	$ 19,754
Q1 2019
Construction Commitment [Line Items]
Committed construction cost (in thousands)	$ 19,754

Construction Commitment, Remaining Minimum Amount Committed

No definition available.

Name:	art_ConstructionCommitmentRemainingMinimumAmountCommitted
Namespace Prefix:	art_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant