Defined Benefit Plan, Average Future Service Period

No definition available.

Name:	art_DefinedBenefitPlanAverageFutureServicePeriod
Namespace Prefix:	art_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Government Sponsored Plans, Contributions

+ References

No definition available.

+ Details

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

- Definition

Multiemployer Plan, Collective-Bargaining Arrangement, Percentage of Employer's Collective-Bargaining Participants Covered

+ References

No definition available.

+ Details

Name:	art_MultiemployerPlanCollectiveBargainingArrangementPercentageofEmployersCollectiveBargainingParticipantsCovered
Namespace Prefix:	art_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Multiemployer Plan, Net Periodic Benefit Cost (Credit)

+ References

No definition available.

+ Details

Name:	art_MultiemployerPlanNetPeriodicBenefitCostCredit
Namespace Prefix:	art_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Multiemployer Plan, Plan Withdrawal Expense

+ References

No definition available.

+ Details

Name:	art_MultiemployerPlanPlanWithdrawalExpense
Namespace Prefix:	art_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Multiemployer Plans, Contribution Term

+ References

No definition available.

+ Details

Name:	art_MultiemployerPlansContributionTerm
Namespace Prefix:	art_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Multiemployer Plans, Funded (Unfunded) Status of Plan

+ References

No definition available.

+ Details

Name:	art_MultiemployerPlansFundedUnfundedStatusofPlan
Namespace Prefix:	art_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Multiemployer Plans, Monthly Contributions

+ References

No definition available.

+ Details

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

- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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

- Definition

Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (s)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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

- Definition

Amount of contributions expected to be received by defined benefit plan from employer in fiscal year. Excludes contributions paid by employer in current fiscal year.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920

+ Details

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

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

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of minimum contribution(s) required for future periods to a multiemployer plan by collective bargaining arrangements, statutory obligations, or other contractual obligations. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (f)(3)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

+ Details

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

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=us-gaap_ForeignPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanNameAxis=art_RetirementIncomePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanNameAxis=art_NationalServiceRelatedPensionPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanNameAxis=art_SuperannuationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MultiemployerPlanNameAxis=art_NewEnglandTeamstersandTruckingIndustryPensionPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MultiemployerPlanTypeAxis=us-gaap_MultiemployerPlansPensionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Employee Benefit Plans - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected contributions to all plans in 2020		$ 2,500,000
Percentage of union employees covered		60.00%
Multiemployer plan expense		$ 18,000,000.0	$ 17,400,000	$ 17,000,000.0
Minimum contribution to multiemployer plan		17,300,000
Multi-employer pension plan withdrawal expense	$ 9,200,000	0	0	9,134,000
Contributions to government sponsored plan		5,800,000	5,700,000	5,400,000
Defined contribution plan expense		4,200,000	$ 3,900,000	$ 3,600,000
U.S. Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected amortization of estimated net gain (loss)		$ (1,600,000)
Expected return on plan assets		6.50%
Offshore Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected amortization of estimated net gain (loss)		$ 0
Expected return on plan assets		5.00%
Pension \| Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Average future service period		6 years 3 months 18 days
Expected return on plan assets		6.50%	7.00%	7.00%
Pension \| National Service-Related Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Average future service period		7 years 4 months 24 days
Expected return on plan assets		6.50%	7.00%	7.00%
Pension \| Superannuation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Average future service period		5 years 9 months 18 days
Expected return on plan assets		5.00%	6.00%	6.00%
Other Post-Retirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Average future service period		5 years
New England Teamsters & Trucking Industry Pension Plan \| Multiemployer Plans, Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded liability on multiemployer plan		$ 13,700,000
Monthly installments on multiemployer plan		$ 40,000.00
Contribution term of multiemployer plan		30 years