Represents the deferred estimated percentage rent payments received during the period.

No definition available.

Name:	snh_DeferredEstimatedPercentageRentReceived
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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 shares held as a percentage of the total shares outstanding at the end of the period.

+ References

No definition available.

+ Details

Name:	snh_InvestmentOwnedPercentageOfTotalSharesOutstanding
Namespace Prefix:	snh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of private pay independent living units within communities acquired.

+ References

No definition available.

+ Details

Name:	snh_NumberOfAcquiredPrivatePayIndependentLivingUnits
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

Represents the number of pooling agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of pre-existing management agreements.

+ References

No definition available.

+ Details

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

- Definition

Number of Pre-Existing Management Agreements Terminated

+ References

No definition available.

+ Details

Name:	snh_NumberOfPreExistingManagementAgreementsTerminated
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.

+ 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

Represents the number of separate management agreements entered for communities acquired, upon termination of a pre-existing combined management agreement.

+ References

No definition available.

+ Details

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

- Definition

Number Of Separate Management Agreements Used by Nine Pooling Agreements

+ References

No definition available.

+ Details

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

- Definition

Number Of Separate Management Agreements Used by One Pooling Agreement

+ References

No definition available.

+ Details

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

- Definition

Number Of Units That May Be Re-Assigned To An Existing Or New LOng Term Lease

+ References

No definition available.

+ Details

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

- Definition

Number of Properties Managed By Significant Tenant

+ References

No definition available.

+ Details

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

- 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 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.

+ References

No definition available.

+ Details

Name:	snh_RealEstateAggregateSalesPrice
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

Represents the business management fees incurred pursuant to business management agreement with related parties.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionBusinessManagementFees
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 the annual return as a percentage of the entity's invested surplus used to determine the incentive fee payable to related parties.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPropertyManagementAgreementAnnualReturnAsPercentageOfInvestedCapital
Namespace Prefix:	snh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the incentive fee payable to related parties under property management agreement expressed as a percentage of the annual net operating income after the entity realizes an annual return equal to specified percentage of its invested capital.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPropertyManagementAgreementIncentiveFeeAsPercentageOfAnnualNetOperatingIncomeAfterRealizationOfSpecifiedPercentageOfAnnualReturn
Namespace Prefix:	snh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the management fees payable to related parties under property management agreement expressed as a percentage of gross revenues.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPropertyManagementAgreementManagementFeesAsPercentageOfGrossRevenues
Namespace Prefix:	snh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of consecutive renewal terms of agreement that the entity has the option to renew.

+ References

No definition available.

+ Details

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

- Definition

Represents the payment of management capital expenditure as a percentage of amount funded by the company.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPropertyManagementAgreementPaymentOfCapitalExpenditureAsPercentageOfFundedAmount
Namespace Prefix:	snh_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Related Party Transaction Property Management Agreement, Payment of Capital Expenditure

+ References

No definition available.

+ Details

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

- Definition

Represents the number of years for which the entity has the option to renew the agreement.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPropertyManagementAgreementRenewalPeriod
Namespace Prefix:	snh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of consecutive period during which the entity must not receive the minimum return for the property management agreement to be subject to the pooling agreement.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPropertyManagementAgreementSubjectToPoolingAgreementMinimumReturnNotReceivedPeriod
Namespace Prefix:	snh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the annual rent as a percentage of the entity's gross revenues.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionTransactionAgreementAnnualRentAsPercentageOfGrossRevenues
Namespace Prefix:	snh_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- 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

Aggregate amount of receivables to be collected from related parties where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth, at the financial statement date. which are usually due after one year (or one business cycle).

+ 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 210.5-02.11)
-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 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Definition

Balance held at close of period in number of shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-12C.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611225-123010

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 6
-Subparagraph (SX 210.12-14.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611322-123010

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-12.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010

+ Details

Name:	us-gaap_InvestmentOwnedBalanceShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
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 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

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=snh_TripleNetSeniorLivingCommunitiesMember
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_StatementGeographicalAxis=stpr_FL
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_StatementGeographicalAxis=stpr_WI
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_GA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_NC
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	snh_RealEstateAgreementsByTypeAxis=snh_NewPoolingAgreementWhichCombineSixALManagementAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	snh_RealEstateAgreementsByTypeAxis=snh_NewPoolingAgreementWhichCombineFiveALManagementAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	snh_RealEstateAgreementsByTypeAxis=snh_ALPoolingAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DiscontinuedOperationsDisposedOfBySaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_IL
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Person Transactions - Five Star (Details)					1 Months Ended		3 Months Ended				9 Months Ended
Related Person Transactions - Five Star (Details)	Oct. 31, 2016 living_unit	Oct. 31, 2016 community	Sep. 30, 2016 USD ($) community shares	Jun. 29, 2016 USD ($) community lease_agreement pooling_agreement management_agreement	Sep. 30, 2016 USD ($) community living_unit shares	May 31, 2016 USD ($)	Dec. 31, 2016 USD ($) living_unit community	Sep. 30, 2016 USD ($) community shares	Mar. 31, 2016 community	Sep. 30, 2015 USD ($) community	Sep. 30, 2016 USD ($) community shares	Sep. 30, 2015 USD ($) community	Apr. 30, 2016 living_unit	Dec. 31, 2015 USD ($)	May 31, 2015	Dec. 31, 2001
Minimum
Related person transactions
Number Of Units That May Be Re-Assigned To An Existing Or To A New Long Term Lease \| community			1		1			1			1
Triple Net Leased Senior Living Communities
Related person transactions
Number of communities \| community									2
Five Star
Related person transactions
Percentage of total shares outstanding			8.60%		8.60%			8.60%			8.60%					100.00%
Investment in common shares (in shares) \| shares			4,235,000		4,235,000			4,235,000			4,235,000
Number of pre-existing agreements \| lease_agreement				4
Number of management agreements used by one pooling agreement \| management_agreement				5
Number of management agreements used by nine pooling agreements \| management_agreement				6
Number of real estate properties leased \| community			183		183			183		178	183	178
Total rental income recognized								$ 50,417,000		$ 47,828,000	$ 146,758,000	$ 143,271,000
Estimated percentage rent payments received								1,367,000		1,371,000	4,228,000	4,194,000
Rents receivable			$ 16,831,000		$ 16,831,000			16,831,000			16,831,000			$ 17,466,000
Payment of capital expenditure								266,000			266,000
Five Star \| Florida
Related person transactions
Business management fees incurred											2,100,000
Five Star \| Senior Living Communities
Related person transactions
Number of properties acquired or agreed to be acquired \| community				7
Purchase price excluding closing costs and working capital adjustments				$ 112,350,000
Initial annual rent				$ 8,426,000
Annual rent percentage (as a percent)				0.04
Number of consecutive renewal terms of agreement \| lease_agreement				2
Renewal period				15 years
Number of pre-existing management agreements terminated \| pooling_agreement				3
Number of new management agreements entered \| pooling_agreement				10
Real estate improvements purchased								3,470,000		7,523,000	15,306,000	16,425,000
Increase or decrease in annual lease rent payable								279,000		604,000	1,228,000	$ 1,322,000
Number of properties acquired, referred to as managed properties \| community												60
Property management agreement expense								3,070,000		$ 2,717,000	$ 8,689,000	$ 7,939,000
Five Star \| Senior Living Communities \| Wisconsin
Related person transactions
Increase or decrease in annual lease rent payable					$ 25,000
Five Star \| Senior Living Communities \| Georgia
Related person transactions
Purchase price excluding closing costs and working capital adjustments						$ 8,400,000
Five Star \| Senior Living Communities \| North Carolina
Related person transactions
Number of living units \| living_unit													288
Five Star \| Living Unit
Related person transactions
Number of acquired living units \| living_unit					1
Five Star \| Living Unit \| Florida
Related person transactions
Purchase price excluding closing costs and working capital adjustments			$ 130,000
Increase or decrease in annual lease rent payable					$ 10,000
Five Star \| New Pooling Agreements Which Combine Six AL Management Agreements
Related person transactions
Number of combination or pooling agreements \| pooling_agreement				9
Five Star \| New Pooling Agreement Which Combine Five AL Management Agreements
Related person transactions
Number of combination or pooling agreements \| pooling_agreement				1
Five Star \| First AL Pooling Agreement
Related person transactions
Management fees as a percentage of gross revenues				5.00%											3.00%
Incentive fee as percentage of the annual net operating income after the entity realizes an annual return equal to 8% of invested capital				20.00%											35.00%
Payment of capital expenditure as a percentage of the amount funded by entity				0.03										0.07
Number of consecutive period during which the entity must not receive the minimum return for the property management agreement to be subject to the pooling agreement				3 years
Minimum annual rent from Five Star														$ 3,610,000
Five Star \| First AL Pooling Agreement \| Minimum
Related person transactions
Annual return as a percentage of invested surplus specified as a base for determining incentive fee															8.00%
Five Star \| First AL Pooling Agreement \| Maximum
Related person transactions
Annual return as a percentage of invested surplus specified as a base for determining incentive fee				7.00%
Five Star \| First AL Pooling Agreement \| Senior Living Communities
Related person transactions
Annual return as a percentage of invested surplus specified as a base for determining incentive fee														7.00%
Discontinued Operations, Disposed of by Sale \| Five Star
Related person transactions
Number of communities \| community					1
Increase or decrease in annual lease rent payable								$ 25,000
Sale price of property sold and agreed to be sold					$ 248,000
Discontinued Operations, Disposed of by Sale \| Five Star \| Wisconsin
Related person transactions
Number of communities \| community					1
Sale price of property sold and agreed to be sold					$ 248,000
Subsequent Event \| Five Star \| Georgia
Related person transactions
Number of properties managed by significant tenant	350	4
Five Star
Related person transactions
Number of properties managed by significant tenant \| community											63
Scenario, Forecast \| Triple Net Leased Senior Living Communities \| Illinois
Related person transactions
Number of properties acquired or agreed to be acquired \| community							2
Number of acquired living units \| living_unit							126
Purchase price excluding closing costs and working capital adjustments							$ 18,600,000