Debt Instrument, Collateral, Shares Pledged

No definition available.

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Namespace Prefix:	art_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Collateral, Shares Pledged, Percentage of Shares Issued and Outstanding

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Percentage

+ References

No definition available.

+ Details

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

- Definition

Notes Payable, Related Party, Percentage

+ References

No definition available.

+ Details

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

- Definition

Related Party Transaction, Committed Number of Shares Owned by Related Party

+ References

No definition available.

+ Details

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Namespace Prefix:	art_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of interest expense incurred on a debt or other obligation to related party.

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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Balance Type:	na
Period Type:	duration

- Details

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- Details

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- Details

Name:	us-gaap_CounterpartyNameAxis=art_AffiliateofGoldmanMember
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- Details

Name:	us-gaap_DebtInstrumentAxis=art_AustraliaTermLoanMember
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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_MediumTermNotesMember
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- Details

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- Details

Name:	us-gaap_CounterpartyNameAxis=art_TheGoldmanSachsGroupInc.Member
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- Details

Name:	us-gaap_DebtInstrumentAxis=art_TwoThousandFifteenSeniorSecuredCreditFacilitiesMemberMember
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Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
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- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=art_YFARTHoldingsL.P.Member
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Related-Party Transactions - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 08, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Par value of common shares (in USD per share)	$ 0.01		$ 0.01	$ 0.01
YF ART Holdings L.P.
Related Party Transaction [Line Items]
Shares pledged in financing arrangement (in shares)			54,952,774
Shares pledged in financing arrangement as a percentage of shares issue and outstanding			38.60%
Principal Owner \| Affiliate of Goldman \| Australia Term Loan \| Term Loans
Related Party Transaction [Line Items]
Lender participation in loans	2.50%
Principal Owner \| Affiliate of Goldman \| New Zealand Term Loan \| Term Loans
Related Party Transaction [Line Items]
Lender participation in loans	31.80%
Principal Owner \| Goldman
Related Party Transaction [Line Items]
Interest expense	$ 0.9	$ 0.3
Principal Owner \| Goldman \| 2015 Revolving Credit Facility \| Credit Facility \| Revolving Credit Facility
Related Party Transaction [Line Items]
Commitments from lending group under credit facility	$ 45.0
Commitments from lending group under credit facility, percentage	4.90%
Principal Owner \| YF ART Holdings L.P.
Related Party Transaction [Line Items]
Number of shares committed to be owned by related party (in shares)			27,500,000.0