Debt - Schedule of Outstanding Borrowings (Details) $ in Thousands	6 Months Ended
Jun. 30, 2018 USD ($) warehouse	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Carrying Amount	$ 1,400,386
Total debt net of unamortized discount and deferred financing costs	1,385,682
Mortgage Notes and Term Loans
Debt Instrument [Line Items]
Carrying Amount	1,400,386	$ 1,753,955
Less unamortized deferred financing costs	(14,395)	(25,712)
Less unamortized debt discount	(309)	(6,285)
Total debt net of unamortized discount and deferred financing costs	1,385,682	1,721,958
Fair Value	$ 1,414,249	1,787,528
Mortgage Loans \| 2010 Mortgage Loans
Debt Instrument [Line Items]
Number of properties \| warehouse	46
Mortgage Loans \| Component A-1
Debt Instrument [Line Items]
Contractual Interest Rate	3.86%
Effective Interest Rate as of June 30, 2018	4.40%
Carrying Amount	$ 48,287	56,941
Fair Value	$ 49,012	58,151
Mortgage Loans \| Component A-2-FX
Debt Instrument [Line Items]
Contractual Interest Rate	4.96%
Effective Interest Rate as of June 30, 2018	5.38%
Carrying Amount	$ 150,334	150,334
Fair Value	$ 156,347	159,918
Mortgage Loans \| Component A-2-FL
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2018	3.96%
Carrying Amount	$ 48,654	48,654
Fair Value	$ 49,019	49,019
Mortgage Loans \| Component A-2-FL \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate	1.51%
Mortgage Loans \| Component B
Debt Instrument [Line Items]
Contractual Interest Rate	6.04%
Effective Interest Rate as of June 30, 2018	6.48%
Carrying Amount	$ 60,000	60,000
Fair Value	$ 63,300	64,875
Mortgage Loans \| Component C
Debt Instrument [Line Items]
Contractual Interest Rate	6.82%
Effective Interest Rate as of June 30, 2018	7.28%
Carrying Amount	$ 62,400	62,400
Fair Value	$ 66,924	68,718
Mortgage Loans \| Component D
Debt Instrument [Line Items]
Contractual Interest Rate	7.45%
Effective Interest Rate as of June 30, 2018	7.92%
Carrying Amount	$ 82,600	82,600
Fair Value	$ 89,208	91,686
Mortgage Loans \| 2013 Mortgage Loans
Debt Instrument [Line Items]
Number of properties \| warehouse	15
Mortgage Loans \| Senior note
Debt Instrument [Line Items]
Contractual Interest Rate	3.81%
Effective Interest Rate as of June 30, 2018	4.14%
Carrying Amount	$ 191,110	194,223
Fair Value	$ 186,810	195,194
Mortgage Loans \| Mezzanine A
Debt Instrument [Line Items]
Contractual Interest Rate	7.38%
Effective Interest Rate as of June 30, 2018	7.55%
Carrying Amount	$ 70,000	70,000
Fair Value	$ 67,200	68,950
Mortgage Loans \| Mezzanine B
Debt Instrument [Line Items]
Contractual Interest Rate	11.50%
Effective Interest Rate as of June 30, 2018	11.75%
Carrying Amount	$ 32,000	32,000
Fair Value	$ 31,040	31,840
Term Loans \| Australia Term Loan
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2018	4.63%
Carrying Amount	$ 150,200	158,645
Fair Value	$ 151,514	160,628
Term Loans \| Australia Term Loan \| BBSY
Debt Instrument [Line Items]
Contractual Interest Rate	1.40%
Term Loans \| New Zealand Term Loan
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2018	5.21%
Carrying Amount	$ 29,801	31,240
Fair Value	$ 30,062	31,631
Term Loans \| New Zealand Term Loan \| BKBM
Debt Instrument [Line Items]
Contractual Interest Rate	1.40%
Term Loans \| 2015 Senior Secured Term Loan B Facility
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2018	5.79%
Carrying Amount	$ 0	806,918
Fair Value	$ 0	806,918
Term Loans \| 2015 Senior Secured Term Loan B Facility \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate	3.75%
Line of Credit \| 2018 Senior Secured Term A Facility secured by stock pledge in qualified subsidiaries
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2018	4.95%
Carrying Amount	$ 475,000	0
Fair Value	$ 473,813	0
Line of Credit \| 2018 Senior Secured Term A Facility secured by stock pledge in qualified subsidiaries \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate	2.35%
Line of Credit \| 2018 Senior Secured Revolving Credit Facility secured by stock pledge in qualified subsidiaries \| Revolving Credit Facility
Debt Instrument [Line Items]
Total debt net of unamortized discount and deferred financing costs	$ 0	0
Fair Value	0	0
Construction Loan
Debt Instrument [Line Items]
Less unamortized deferred financing costs		$ (179)
Construction Loan \| Warehouse Clearfield, UT secured by mortgage
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2018		5.18%
Carrying Amount	0	$ 19,671
Less unamortized deferred financing costs	0	(179)
Total debt net of unamortized discount and deferred financing costs	0	19,492
Fair Value	$ 0	$ 19,671
Construction Loan \| Warehouse Clearfield, UT secured by mortgage \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate	3.25%

Debt Instrument, Collateral Number of Properties

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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

Reference 2: 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(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: 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

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

+ Details

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

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DeferredFinanceCostsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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