Collective-Bargaining Arrangement, Number of Agreements which are Expiring, Next Twelve Months

No definition available.

Name:	art_CollectiveBargainingArrangementNumberofAgreementswhichareExpiringNextTwelveMonths
Namespace Prefix:	art_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Collective-Bargaining Arrangement, Number of Warehouses where Agreements are Expiring, Next Twelve Months

+ References

No definition available.

+ Details

Name:	art_CollectiveBargainingArrangementNumberofWarehouseswhereAgreementsareExpiringNextTwelveMonths
Namespace Prefix:	art_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Collective-Bargaining Agreements

+ References

No definition available.

+ Details

Name:	art_NumberofCollectiveBargainingAgreements
Namespace Prefix:	art_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Collective-Bargaining Agreements, Including Agreements Under Negotiation

+ References

No definition available.

+ Details

Name:	art_NumberofCollectiveBargainingAgreementsIncludingAgreementsUnderNegotiation
Namespace Prefix:	art_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of persons employed by the Entity

+ References

No definition available.

+ Details

Name:	dei_EntityNumberOfEmployees
Namespace Prefix:	dei_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount awarded to other party in judgment or settlement of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmountAwardedToOtherParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14435-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12021-110248

+ Details

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_LitigationCaseAxis=art_KansasBreachofSettlementAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_LitigationCaseAxis=art_JoseContrerasEmploymentPutativeClassActionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_SuretyBondMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_UnionizedEmployeesConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_NumberOfEmployeesTotalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (Details) $ in Millions		12 Months Ended
Commitments and Contingencies (Details) $ in Millions	Feb. 22, 2019 USD ($)	Dec. 31, 2019 USD ($) employee warehouse agreement	Dec. 31, 1994 USD ($)	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Letter of credit amount outstanding		$ 23.0		$ 29.6
Number of employees \| employee		12,600
Number of warehouses \| warehouse		178
Number of collective bargaining agreements \| agreement		74
Number of collective bargaining agreements, including agreements under negotiation \| agreement		75
Number of collective bargaining agreements expiring in 2020 \| agreement		19
Number of warehouses where collective bargaining agreements are expiring in 2020 \| warehouse		26
Kansas Breach of Settlement Agreement
Loss Contingencies [Line Items]
Litigation settlement, awarded to other party			$ 58.7
Jose Contreras Employment Putative Class Action
Loss Contingencies [Line Items]
Litigation settlement, awarded to other party	$ 2.5
Surety Bond
Loss Contingencies [Line Items]
Outstanding surety bond		$ 4.3		$ 2.7
Unionized Employees Concentration Risk
Loss Contingencies [Line Items]
Number of warehouses \| warehouse		84
Number of Employees \| Unionized Employees Concentration Risk
Loss Contingencies [Line Items]
Collective-bargaining arrangement, percentage of participants		49.00%