Represent the amounts due from related person, owed to us from SIR related to working capital activity for MOBs.

No definition available.

Name:	snh_AmountOwedFromSir
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the deferred percentage rent estimated during the period.

+ References

No definition available.

+ Details

Name:	snh_DeferredEstimatedPercentageRent
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the increase or decrease on the annual rent the lessee is obligated to pay on an operating lease.

+ References

No definition available.

+ Details

Name:	snh_IncreaseDecreaseOperatingLeasesAnnualRent
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of communities.

+ References

No definition available.

+ Details

Name:	snh_NumberOfCommunities
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the number of properties acquired or agreed to be acquired by the entity during the period.

+ References

No definition available.

+ Details

Name:	snh_NumberOfPropertiesAcquired
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of properties acquired or agreed to be acquired by the entity during the period, which are referred by the entity as managed properties.

+ References

No definition available.

+ Details

Name:	snh_NumberOfPropertiesAcquiredManagedProperties
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the number of properties sold or agreed to be sold by the entity.

+ References

No definition available.

+ Details

Name:	snh_NumberOfPropertiesSold
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of real estate properties serving as collateral for debt as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	snh_NumberOfRealEstatePropertiesCollateralized
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of real estate properties owned by the entity, which are classified as held for sale as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	snh_NumberOfRealEstatePropertiesHeldForSale
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the annual rent the lessee is obligated to pay on an operating lease.

+ References

No definition available.

+ Details

Name:	snh_OperatingLeasesAnnualRent
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the aggregate purchase price excluding closing costs, of real estate properties acquired by the entity during the period.

+ References

No definition available.

+ Details

Name:	snh_RealEstateAggregatePurchasePrice
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the aggregate purchase price of real estate properties acquired by the entity during the period. This amount includes cash paid plus debt assumed, if any, and other settlement adjustments but excludes closing costs.

+ References

No definition available.

+ Details

Name:	snh_RealEstateAggregatePurchasePriceIncludingOtherSettlementAdjustments
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the aggregate sales price excluding closing costs, of real estate properties sold by the entity during the period.

+ References

No definition available.

+ Details

Name:	snh_RealEstateAggregateSalesPrice
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the debt disposed of in sale of property transactions.

+ References

No definition available.

+ Details

Name:	snh_RealEstateDebtDisposedOfInSaleOfPropertyTransactions
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of improvements to real estate properties made by lessees and purchased by the entity.

+ References

No definition available.

+ Details

Name:	snh_RealEstateImprovementsByLesseePurchased
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionExpensesFromBusinessManagementAgreementTransactionsWithRelatedParty
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionExpensesFromPropertyManagementAgreementTransactionsWithRelatedParty
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents rents receivable at the balance sheet date.

+ References

No definition available.

+ Details

Name:	snh_RentsReceivable
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
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 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

-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 323

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

+ 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=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 9

-Article 5

Reference 3: 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 4: 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 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 amount of interest income and other income recognized during the period. Included in this element is interest derived from investments in debt securities, cash and cash equivalents, and other investments which reflect the time value of money or transactions in which the payments are for the use or forbearance of money and other income from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business).

+ References

No definition available.

+ Details

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

- Definition

The number of states the entity operates in as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfStatesInWhichEntityOperates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of units in a real estate property owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfUnitsInRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The total amount of revenue recognized for the period from operating leases, including minimum lease revenue, contingent revenue, percentage revenue and sublease revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=7501430&loc=d3e39896-112707

+ Details

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

- Definition

The amount of revenue recognized for the period under subleasing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 55

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6584307&loc=d3e41620-112719

+ Details

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

- Definition

The number of units (items of property) under operating lease arrangements.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertySubjectToOrAvailableForOperatingLeaseNumberOfUnits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 3: 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

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=snh_MedicalOfficeBuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=snh_SeniorLivingCommunityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis=snh_AgreementToAcquireMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis=us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended		3 Months Ended			12 Months Ended	1 Months Ended	0 Months Ended	1 Months Ended
	Feb. 28, 2015 property	Apr. 30, 2014	Mar. 31, 2015 property item	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2014 community item	Apr. 30, 2015	May 01, 2015	Jan. 31, 2015 property sqft	May 31, 2015 community	Dec. 31, 2014 community item
Related person transactions
Issuance of shares (in shares)	31,050,000	15,525,000		31,050,000
Number of states in which properties are located			39
Income (loss) recognized arising from equity investments			$ 72		$ (97)
Number of properties agreed to be offered for sale			4
Number of properties sold or agreed to be sold	1snh_NumberOfPropertiesSold
Interest and other income			75us-gaap_InterestAndOtherIncome		105us-gaap_InterestAndOtherIncome
Mortgages
Related person transactions
Number of Real Estate Properties Collateralized			46
Senior living communities
Related person transactions
Number of living units			192			395					395
Number Of Communities						5
Number of properties agreed to be offered for sale			3
Senior living communities \| Pennsylvania
Related person transactions
Sale price of properties under agreement to be sold	250
MOBs
Related person transactions
Sale price of properties under agreement to be sold							1,500
Number of properties agreed to be offered for sale			1		4
Acquisition
Related person transactions
Total rental income recognized			7,046
Number of properties acquired or agreed to be acquired			23
Acquisition \| Senior living communities
Related person transactions
Number of properties acquired or agreed to be acquired						2
Acquisition \| Senior living communities \| Subsequent event
Related person transactions
Number of living units								3,379
Purchase price								762,611
Acquisition \| MOBs
Related person transactions
Purchase price excluding closing costs			539,000						539,000
Number of properties acquired or agreed to be acquired			23			2			23
Purchase price									539,000
Area of real estate properties (in square feet)			2,170,000						2,170,000
Agreement to acquire \| Senior living communities
Related person transactions
Number of living units						3,466					3,466
Number Of Communities										37	38
Number of properties acquired or agreed to be acquired						38
Purchase price						790,000
Agreement to acquire \| Senior living communities \| GA
Related person transactions
Number of living units							40
Number Of Communities							1
Purchase price							9,750
Five Star
Related person transactions
Minimum annual rent payable to entity			191,007
Total rental income recognized			47,691		47,506
Rents receivable			15,893			17,310					17,310
Deferred estimated percentage rent			1,456		1,416
Real estate improvements purchased			4,550		8,614
Increase or decrease in annual lease rent payable			367		689
Sublease rental			752		730
Five Star \| Senior living communities
Related person transactions
Number of real estate properties leased			180
Number of living units						881					881
Number of properties acquired, referred to as managed properties			46
Property management agreement expense			2,523		2,425
Number Of Communities						14
Five Star \| Senior living communities \| GA
Related person transactions
Number of living units							40
Purchase price excluding closing costs							9,750
Five Star \| Acquisition \| Senior living communities
Related person transactions
Number of living units										881
Number Of Communities										14
Five Star \| Sale \| Senior living communities \| Pennsylvania
Related person transactions
Increase or decrease in annual lease rent payable	23
Number of living units	120
Sale price of properties under agreement to be sold	250
RMR
Related person transactions
Property management agreement expense			2,438		1,638
Business management agreement expense			8,869		6,682
Issuance of shares (in shares)			39,467
SIR
Related person transactions
Debt disposed of from properties sold									29,955
Amount from SIR related to working capital activity			8,993
SIR \| Acquisition \| MOBs
Related person transactions
Number of properties acquired or agreed to be acquired									23
Purchase price									539,000
Area of real estate properties (in square feet)									2,170,000
Number of states in which properties are located			12
AIC
Related person transactions
Equity method investments, carrying value			6,944
Income (loss) recognized arising from equity investments			$ 72		$ (97)