Line of Credit Facility, Extension Term

No definition available.

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

- Definition

Line of Credit Facility, Number of Extensions

+ References

No definition available.

+ Details

Name:	art_LineofCreditFacilityNumberofExtensions
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

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
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 of unamortized debt discount (premium) and debt issuance costs.

+ References

+ Details

Name:	us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet
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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120398452&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&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

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

Debt (Details)						6 Months Ended
Debt (Details)	Mar. 26, 2020 USD ($) extension	Sep. 24, 2019	Sep. 23, 2019	Dec. 04, 2018	Dec. 03, 2018	Jun. 30, 2020 USD ($)	Jun. 30, 2020 CAD ($)	Mar. 26, 2020 CAD ($) extension	Dec. 31, 2019 USD ($)	Apr. 26, 2019 USD ($)	Nov. 06, 2018 USD ($)	May 01, 2013 USD ($)
Mortgages, Senior Notes and Term Loans
Debt Instrument [Line Items]
Carrying Amount						$ 1,838,701,000			$ 1,708,443,000
Estimated Fair Value						2,002,183,000			1,783,463,000
Less: unamortized deferred financing costs						(14,295,000)			(12,996,000)
Total debt net of unamortized deferred financing costs						1,824,406,000			1,695,447,000
Mortgage Loans \| 2013 Mortgage Loans
Debt Instrument [Line Items]
Carrying Amount						280,101,000			283,443,000
Estimated Fair Value						$ 285,958,000			287,463,000
Face amount of debt												$ 322,000,000.0
Mortgage Loans \| Senior note
Debt Instrument [Line Items]
Contractual Interest Rate						3.81%	3.81%
Effective Interest Rate as of June 30, 2020						4.14%	4.14%
Carrying Amount						$ 178,101,000			181,443,000
Estimated Fair Value						$ 182,108,000			184,618,000
Mortgage Loans \| Mezzanine A
Debt Instrument [Line Items]
Contractual Interest Rate						7.38%	7.38%
Effective Interest Rate as of June 30, 2020						7.55%	7.55%
Carrying Amount						$ 70,000,000			70,000,000
Estimated Fair Value						$ 71,050,000			70,525,000
Mortgage Loans \| Mezzanine B
Debt Instrument [Line Items]
Contractual Interest Rate						11.50%	11.50%
Effective Interest Rate as of June 30, 2020						11.75%	11.75%
Carrying Amount						$ 32,000,000			32,000,000
Estimated Fair Value						32,800,000			32,320,000
Senior Notes \| Senior Unsecured Notes
Debt Instrument [Line Items]
Carrying Amount						950,000,000			950,000,000
Estimated Fair Value						$ 1,107,625,000			1,023,375,000
Senior Notes \| Series A 4.68% notes due 2026
Debt Instrument [Line Items]
Contractual Interest Rate						4.68%	4.68%				4.68%
Effective Interest Rate as of June 30, 2020						4.77%	4.77%
Carrying Amount						$ 200,000,000			200,000,000
Estimated Fair Value						$ 229,000,000			217,750,000
Face amount of debt											$ 200,000,000.0
Senior Notes \| Series B 4.86% notes due 2029
Debt Instrument [Line Items]
Contractual Interest Rate						4.86%	4.86%				4.86%
Effective Interest Rate as of June 30, 2020						4.92%	4.92%
Carrying Amount						$ 400,000,000			400,000,000
Estimated Fair Value						$ 477,000,000			439,000,000
Face amount of debt											$ 400,000,000.0
Senior Notes \| Series C 4.10% notes due 2030
Debt Instrument [Line Items]
Contractual Interest Rate						4.10%	4.10%			4.10%
Effective Interest Rate as of June 30, 2020						4.15%	4.15%
Carrying Amount						$ 350,000,000			350,000,000
Estimated Fair Value						401,625,000			366,625,000
Face amount of debt										$ 350,000,000.0
Term Loans \| Total 2020 Senior Unsecured Term Loan A Facility
Debt Instrument [Line Items]
Carrying Amount						608,600,000			0
Estimated Fair Value						$ 608,600,000			0
Term Loans \| 2020 Senior Unsecured Term Loan Tranche A-1
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2020						2.65%	2.65%
Carrying Amount						$ 425,000,000			0
Estimated Fair Value						$ 425,000,000			0
Face amount of debt	$ 425,000,000.0
Term Loans \| 2020 Senior Unsecured Term Loan Tranche A-1 \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate						0.95%
Term Loans \| 2020 Senior Unsecured Term Loan Tranche A-2
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2020						1.61%	1.61%
Carrying Amount						$ 183,600,000			0
Estimated Fair Value						$ 183,600,000			0
Face amount of debt							$ 250,000,000.0	$ 250,000,000.0
Term Loans \| 2020 Senior Unsecured Term Loan Tranche A-2 \| CDOR
Debt Instrument [Line Items]
Contractual Interest Rate						0.95%
Term Loans \| 2018 Senior Unsecured Term Loan A Facility
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2020						3.14%	3.14%
Carrying Amount						$ 0			475,000,000
Estimated Fair Value						$ 0			472,625,000
Face amount of debt	$ 475,000,000
Term Loans \| 2018 Senior Unsecured Term Loan A Facility \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate						1.00%
Revolving Credit Facility \| Line of Credit \| 2018 Senior Unsecured Term Loan A Facility \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate		1.00%	1.45%	1.45%	2.35%
Revolving Credit Facility \| Line of Credit \| 2020 Senior Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2020						0.23%	0.23%
Carrying Amount						$ 0
Estimated Fair Value						$ 0
Number of extensions \| extension	2							2
Extension term	6 months
Revolving Credit Facility \| Line of Credit \| 2020 Senior Unsecured Revolving Credit Facility \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate						0.85%
Revolving Credit Facility \| Line of Credit \| 2018 Senior Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Effective Interest Rate as of June 30, 2020						0.36%	0.36%
Carrying Amount									0
Estimated Fair Value									$ 0
Revolving Credit Facility \| Line of Credit \| 2018 Senior Unsecured Revolving Credit Facility \| LIBOR
Debt Instrument [Line Items]
Contractual Interest Rate						0.90%