Debt Instrument, Assets Used in Calculation of Collateral Amount

No definition available.

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- Definition

Debt Instrument, Covenant, Base Coverage Ratio, Minimum

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenant, Borrowing Base Debt Service Coverage Ratio, Minimum

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenant, Fixed Charge Coverage Ratio, Minimum

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenant, Leverage Ratio, Maximum

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Debt Instrument, Covenant, Tangible Net Worth, Additional Percentage of Net Equity Proceeds, Minimum

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Debt Instrument, Covenant, Tangible Net Worth, Minimum

+ References

No definition available.

+ Details

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Balance Type:	debit
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- Definition

Debt Instrument, Extension Term

+ References

No definition available.

+ Details

Name:	art_DebtInstrumentExtensionTerm
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Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Recourse Secured Debt Ratio, Maximum

+ References

No definition available.

+ Details

Name:	art_DebtInstrumentRecourseSecuredDebtRatioMaximum
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Balance Type:	na
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- Definition

Line of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

Name:	art_LineOfCreditFacilityAccordionFeatureIncreaseLimit
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Increase (Decrease) Maximum Borrowing Capacity

+ References

No definition available.

+ Details

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

- Definition

Line Of Credit Facility, Threshold For Commitment Fee

+ References

No definition available.

+ Details

Name:	art_LineOfCreditFacilityThresholdForCommitmentFee
Namespace Prefix:	art_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
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Balance Type:	na
Period Type:	duration

- Definition

Amount of assets pledged to secure a debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

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

- Definition

Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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Balance Type:	credit
Period Type:	duration

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Debt - Additional Information (Details)				9 Months Ended
Debt - Additional Information (Details)	Mar. 31, 2018	Feb. 06, 2018 USD ($)	Jan. 23, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Debt				$ 1,376,998,000
Line of Credit \| 2018 Senior Secured Credit Facilities
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 925,000,000
Additional accordion option			$ 400,000,000.0
Gross value of assets included in calculations under credit agreement				1,800,000,000
Borrowing base collateral value				$ 1,100,000,000
Maximum leverage ratio				60.00%
Minimum base coverage ratio				1.00
Minimum fixed charge coverage ratio	1.50			1.40
Minimum borrowing base debt service coverage ratio				2.00
Minimum tangible net worth requirement				$ 900,000,000
Minimum tangible net worth requirement, additional percentage of net equity proceeds				70.00%
Maximum recourse secured debt ratio				0.2
Line of Credit \| 2018 Senior Secured Credit Facilities \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage				0.30%
Commitment threshold				50.00%
Line of Credit \| 2018 Senior Secured Credit Facilities \| Maximum
Debt Instrument [Line Items]
Commitment fee percentage				0.40%
Commitment threshold				50.00%
Line of Credit \| 2018 Senior Secured Credit Facilities \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Line of Credit \| 2018 Senior Secured Credit Facilities \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate				2.35%
Line of Credit \| 2018 Senior Secured Credit Facilities \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate				3.00%
Line of Credit \| 2018 Senior Secured Credit Facilities \| Base Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate				1.35%
Line of Credit \| 2018 Senior Secured Credit Facilities \| Base Rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate				2.00%
Line of Credit \| Senior Secured Term Loan A Facility
Debt Instrument [Line Items]
Term of debt			5 years
Line of credit facility, maximum borrowing capacity			$ 525,000,000.0
Proceeds from lines of credit			$ 517,000,000
Repayments of lines of credit		50,000,000
Line of Credit \| Senior Secured Term Loan A Facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate				2.35%
Line of Credit \| Revolving Credit Facility \| 2018 Senior Secured Revolving Credit Facility secured by stock pledge in qualified subsidiaries
Debt Instrument [Line Items]
Term of debt			3 years
Line of credit facility, maximum borrowing capacity		450,000,000	$ 400,000,000
Extension term			1 year
Increase in credit commitments		$ 50,000,000
Debt				$ 0		$ 0
Line of Credit \| Letter of Credit \| 2018 Senior Secured Credit Facilities
Debt Instrument [Line Items]
Repayments of lines of credit			$ 33,600,000
Debt				$ 32,700,000	$ 33,600,000
Term Loans \| 2015 Senior Secured Term Loan B Facility
Debt Instrument [Line Items]
Repayments of lines of credit			806,900,000
Term Loans \| 2015 Senior Secured Term Loan B Facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate				3.75%
Construction Loan \| Clearfield, Utah and Middleboro Massachusetts Construction Loans
Debt Instrument [Line Items]
Indebtedness repaid			$ 20,900,000