Represents the expense amount accrued for employee share grants and internal audit costs to RMR LLC employees.

No definition available.

Name:	sir_AccrualForEmployeeShareGrants
Namespace Prefix:	sir_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the quarterly dividend payable for each share of common stock outstanding.

+ References

No definition available.

+ Details

Name:	sir_DividendsPayableAmountPerSharePerQuarter
Namespace Prefix:	sir_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the initial other liabilities recorded which is the difference of the total consideration paid and the fair market value of the investment.

+ References

No definition available.

+ Details

Name:	sir_InitialOtherLiabilities
Namespace Prefix:	sir_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the duration of continuing terms of the management agreement.

+ References

No definition available.

+ Details

Name:	sir_ManagementAgreementContinuingTerms
Namespace Prefix:	sir_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of related entities under common management with an equity interst in an affiliate entity.

+ References

No definition available.

+ Details

Name:	sir_NumberOfRelatedEntitiesWithEquityInterestInAffiliate
Namespace Prefix:	sir_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Total expense amount to reimburse a related party.

+ References

No definition available.

+ Details

Name:	sir_RelatedPartyReimbursementExpense
Namespace Prefix:	sir_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of amortization of the liability recorded during the period.

+ References

No definition available.

+ Details

Name:	sir_RelatedPartyTransactionAmortizationOfLiability
Namespace Prefix:	sir_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the property management and construction supervision fees incurred pursuant to business and property management agreements with related parties.

+ References

No definition available.

+ Details

Name:	sir_RelatedPartyTransactionPropertyManagementAndConstructionSupervisionFees
Namespace Prefix:	sir_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the incentive fee payable in a related party transaction.

+ References

No definition available.

+ Details

Name:	sir_Relatedpartytransactionincentivefeepayable
Namespace Prefix:	sir_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

+ Details

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

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

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 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.12)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 9
-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Paragraph 11
-Article 7

+ Details

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

- Definition

The change in other comprehensive income during the reporting period, net of tax, for an item that is not separately disclosed.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Definition

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Related Parties
-URI http://asc.fasb.org/extlink&oid=16382449

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606610-122999

+ Details

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

- Definition

Number of shares issued in lieu of cash for services contributed to the entity. Number of shares includes, but is not limited to, shares issued for services contributed by vendors and founders.

+ References

No definition available.

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesIssuedForServices
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Cumulative number of shares issued or sold by the subsidiary or equity method investee on all stock transactions.

+ References

No definition available.

+ Details

Name:	us-gaap_SubsidiaryOrEquityMethodInvesteeCumulativeNumberOfSharesIssuedForAllTransactions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=sir_GovernmentPropertiesIncomeTrustMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sir_ReitManagementAndResearchIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sir_ReitManagementAndResearchLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=sir_ReitManagementAndResearchLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=sir_AffiliatesInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquityInterestIssuedOrIssuableByTypeAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=sir_ReitManagementAndResearchIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Person Transactions - Up-C Transaction (Details) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended
	Apr. 13, 2016 $ / shares	Jun. 30, 2015 agreement	Mar. 31, 2016 USD ($) agreement shares	Mar. 31, 2015 USD ($) shares	Apr. 25, 2016 $ / shares	Dec. 31, 2015 $ / shares
Related Party Transaction, Due from (to) Related Party [Abstract]
Quarterly dividend payable on common stock (in dollars per share) \| $ / shares					$ 0.50
Income (Loss) from Equity Method Investments			$ 77	$ 72
Equity in unrealized gain of an investee			$ 52	45
GOV
Related Party Transaction, Due from (to) Related Party [Abstract]
Percentage of interest, equity method			27.90%
RMR Inc
Related Party Transaction, Due from (to) Related Party [Abstract]
Number of related entities with an equity interest in an affiliate entity \| agreement		3
RMR LLC
Related Party Transaction, Due from (to) Related Party [Abstract]
Business management fees incurred			$ 5,007	$ 4,646
Shares issued under the business management agreement \| shares				18,656
Property management and construction supervision fees incurred			3,114	$ 2,625
Related party reimbursement expenses			1,621	880
Accrual for RMR LLC employee share grants and internal audit costs			339	235
Rental income earned			5	5
Management agreement continuing terms (in years)		20 years
RMR LLC
Related Party Transaction, Due from (to) Related Party [Abstract]
Incentive fees payable			0	0
AIC
Related Party Transaction, Due from (to) Related Party [Abstract]
Investment at carrying value			6,956
Income (Loss) from Equity Method Investments			77	72
Equity in unrealized gain of an investee			$ 52	$ 45
Number of related entities with an equity interest in an affiliate entity \| agreement			6
Class A common shares
Related Party Transaction, Due from (to) Related Party [Abstract]
Shares holding \| shares			1,586,836
Class A common shares \| RMR Inc
Related Party Transaction, Due from (to) Related Party [Abstract]
Recognized amortization of the liability			$ 558
Related Party Transaction Amortization Of Liability			558
Initial other liabilities			$ 44,072
Shares holding \| shares			1,586,836
Class A common shares \| RMR Inc
Related Party Transaction, Due from (to) Related Party [Abstract]
Distributions per share paid or payable (in dollars per share) \| $ / shares	$ 0.2993
Quarterly dividend payable on common stock (in dollars per share) \| $ / shares	$ 0.25					$ 0.0493