Decrease in Derivative, Amount of Hedged Liability

No definition available.

Name:	art_DecreaseInDerivativeAmountOfHedgedLiability
Namespace Prefix:	art_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Derivative, Amount of Hedged Liability

+ References

No definition available.

+ Details

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

- Definition

Derivatives, Number of Instruments Terminated

+ References

No definition available.

+ Details

Name:	art_DerivativesNumberOfInstrumentsTerminated
Namespace Prefix:	art_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after tax, of accumulated gain (loss) on derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-11

+ Details

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

- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480870/815-30-50-1

+ Details

Name:	us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

+ Details

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

- Definition

The cash outflow for a financial contract that meets the hedge criteria as either a cash flow hedge, fair value hedge, or hedge of a net investment in a foreign operation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 27
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-27

+ Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=art_SeriesDAndSeriesENotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=art_AustralianIntercompanyLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=art_IntercompanyLoanPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=art_TwentyTwentySeniorUnsecuredRevolvingCreditFacilitiesTranche1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=art_TwentyTwentySeniorUnsecuredRevolvingCreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments - Narrative (Details) € in Millions, £ in Millions, $ in Millions, $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2020 USD ($) instrument	Jun. 30, 2024 AUD ($)	Jun. 30, 2024 EUR (€)	Dec. 31, 2023 AUD ($)	Dec. 31, 2023 EUR (€)	Dec. 31, 2023 GBP (£)
Derivative [Line Items]
Decrease in amount of hedged loan	$ 10.4
Number of instruments terminated \| instrument				2
Interest expense
Derivative [Line Items]
Loss to be reclassified in next twelve months			$ 12.9
Foreign Exchange Forward
Derivative [Line Items]
Loss to be reclassified in next twelve months			$ 0.3
Series D and Series E Notes \| Senior Notes
Derivative [Line Items]
Amount of hedged loan					$ 197.0	€ 820.5	$ 191.0	€ 817.5	£ 78.0
Australian Intercompany Loan \| Intercompany Loan Payable \| Foreign Exchange Forward
Derivative [Line Items]
Amount of hedged loan					$ 153.5		$ 153.5
2020 Senior Unsecured Revolving Credit Facility \| Credit Facility \| Interest Rate Swap \| Revolving Credit Facility
Derivative [Line Items]
Loss to be reclassified in next twelve months		$ 0.2
Twenty Twenty Senior Unsecured Revolving Credit Facilities \| Credit Facility \| Revolving Credit Facility
Derivative [Line Items]
Payments for settlement of hedge				$ 16.4
Fee for termination of derivative instruments remaining in accumulated other comprehensive income				$ 8.7