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

No definition available.

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

- Definition

Represents the minimum percentage of common stock held in the investee above which the entity would have the option to cancel all its rights.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of agreements with a related party that is to provide management services to the entity.

+ References

No definition available.

+ Details

Name:	snh_NumberOfAgreementsWithRelatedPartyToProvideManagementServicesToEntity
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 communities managed by a related party.

+ References

No definition available.

+ Details

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

- 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

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 Communities Associated With Leases in Default

+ References

No definition available.

+ Details

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

- Definition

Number of Communities Included in Expansion and Development Project

+ References

No definition available.

+ Details

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

- Definition

Number of Communities Leased by the Company

+ References

No definition available.

+ Details

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

- Definition

Number of Communities Owned and Managed by Related Party

+ References

No definition available.

+ Details

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

- Definition

Number of Communities Owned and Managed by Related Party, Acquired During the Period

+ References

No definition available.

+ Details

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

- Definition

Number of Communities not Included in Pooling Agreement

+ References

No definition available.

+ Details

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

- Definition

Number of Leases With Related Party

+ References

No definition available.

+ Details

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

- Definition

Number of Management Agreements

+ References

No definition available.

+ Details

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

- 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

Related Party Transaction, Annual Rent Increase Percentage in Addition to Rate Applicable on Outstanding Borrowings

+ References

No definition available.

+ Details

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

- Definition

Related Party Transaction, Annual Rents Due

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionAnnualRentsDue
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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 incentive fee payable in a related party transaction.

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionIncentiveFeePayable
Namespace Prefix:	snh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Related Party Transaction, Percentage Of Borrowings Per Annum Under Revolving Credit Facility

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionPercentageOfBorrowingsPerAnnumUnderRevolvingCreditFacility
Namespace Prefix:	snh_
Data Type:	num:percentItemType
Balance Type:	na
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

Related Party Transaction, Property Management Agreement, Number of Communities Subject to Reset of Annual Minimum Return

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

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

+ References

No definition available.

+ Details

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

- Definition

Related Party Transaction, Transaction Agreement Annual Rent as a Percentage of Gross Revenues, Amount

+ References

No definition available.

+ Details

Name:	snh_RelatedPartyTransactionTransactionAgreementAnnualRentasaPercentageofGrossRevenuesAmount
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

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902458&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

- Details

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

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=snh_SeniorLivingCommunityMember
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:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_GA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_NY
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=snh_LeaseNo.1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=snh_LeaseNo.2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=snh_LeaseNo.3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=snh_LeaseNo.4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=snh_LeaseNo.5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_StatementGeographicalAxis=snh_AlabamaAndIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_TN
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_AZ
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Leases and Management Agreements with Five Star - Narrative (Details) $ in Thousands		1 Months Ended					12 Months Ended
	Jun. 29, 2016 community pooling_agreement agreement	Jun. 30, 2018 living_unit	Nov. 30, 2017 community property pooling_agreement renewal_option	Dec. 31, 2016 USD ($) community living_unit	Dec. 31, 2016 USD ($) community living_unit	Jun. 30, 2016 pooling_agreement lease_agreement	Dec. 31, 2018 USD ($) community living_unit	Dec. 31, 2018 USD ($) community living_unit	Dec. 31, 2018 USD ($) community living_unit	Dec. 31, 2018 USD ($) community living_unit lease_agreement	Dec. 31, 2018 USD ($) community living_unit management_agreement	Dec. 31, 2017 USD ($) community	Dec. 31, 2016 USD ($) community living_unit	Nov. 30, 2018 living_unit	Nov. 09, 2017	Jun. 30, 2017 community	Dec. 31, 2015
Five Star
Concentration Risk [Line Items]
Number of real estate properties leased				185	185		184	184	184	184	184	185	185
Total rental income recognized \| $								$ 212,622				$ 210,539	$ 203,581
Rents receivable \| $				$ 18,320	$ 18,320		$ 18,697	$ 18,697	$ 18,697	$ 18,697	$ 18,697	18,539	18,320
Number of units \| living_unit														238
Number of units that may be re-assigned to an existing or to a new long term lease							1	1	1	1	1
Real estate improvements by lessee purchased \| $								$ 17,956				39,800	21,438
Increase in annual lease rent payable \| $								1,433				3,193	1,719
Number of communities included in expansion and development project																2
Percentage in addition to rate applicable on outstanding borrowings																2.00%
Expenses from property management agreement transactions with related party \| $								14,426				$ 14,080	$ 11,918
Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Number of consecutive renewal terms of agreement										2	2
Property management agreement renewal period			3 years				15 years
Number of communities managed									76			70	68
Number of pre-existing management agreements terminated \| pooling_agreement	3					3
Number of pre-existing management agreements	4					4
Number of management agreements	10
Property management agreement, number of communities subject to reset of annual minimum return \| agreement	10
Number of agreements with related party to provide management services to entity \| lease_agreement										2
Expenses from property management agreement transactions with related party \| $								$ 6,442				$ 7,525	$ 7,707
A L Pooling Agreements \| Five Star
Concentration Risk [Line Items]
Number of consecutive renewal terms of agreement \| renewal_option			2
Property management agreement renewal period			15 years
Number of units \| living_unit		98
Percentage of borrowings per annum		2.00%
Percentage in addition to rate applicable on outstanding borrowings			2.00%													2.00%
Management fees as a percentage of gross revenues			5.00%
Incentive fee as a percentage of annual net operating income after realization of specified percentage of annual return			20.00%
Property management agreement, annual return as a percentage of invested capital	7.00%	7.00%	7.00%
Property management agreement, payment of capital expenditure as percentage of the funded amount			3.00%												3.00%		7.00%
Number of communities owned and managed by related party acquired during the period \| property			1
Minimum \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Property management agreement renewal period							10 years
Minimum \| A L Pooling Agreements \| Five Star
Concentration Risk [Line Items]
Management fees as a percentage of gross revenues	3.00%						3.00%	3.00%	3.00%	3.00%	3.00%
Incentive fee as a percentage of annual net operating income after realization of specified percentage of annual return	20.00%						20.00%	20.00%	20.00%	20.00%	20.00%
Property management agreement, annual return as a percentage of invested capital							7.00%	7.00%	7.00%	7.00%	7.00%
Maximum \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Property management agreement renewal period							15 years
Maximum \| A L Pooling Agreements \| Five Star
Concentration Risk [Line Items]
Management fees as a percentage of gross revenues	5.00%						5.00%	5.00%	5.00%	5.00%	5.00%
Incentive fee as a percentage of annual net operating income after realization of specified percentage of annual return	35.00%						35.00%	35.00%	35.00%	35.00%	35.00%
Property management agreement, annual return as a percentage of invested capital							8.00%	8.00%	8.00%	8.00%	8.00%
California
Concentration Risk [Line Items]
Number of communities not included in pooling agreement									1
Georgia \| Senior Living Communities
Concentration Risk [Line Items]
Number of communities					5
Georgia \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Number of communities associated with leases in default				395	5
New York
Concentration Risk [Line Items]
Management fees as a percentage of gross revenues							5.00%	5.00%	5.00%	5.00%	5.00%
Number of communities owned and managed by related party \| living_unit							310	310	310	310	310
Incentive fee payable \| $							$ 0	$ 0	$ 0	$ 0	$ 0
Five Star
Concentration Risk [Line Items]
Number of communities leased by the company				185	185							185	185
Number of leases with related party \| lease_agreement										5
Annual rent as a percentage of gross revenues, percent							4.00%	4.00%	4.00%	4.00%	4.00%
Annual rent as a percentage of gross revenues, amount \| $								$ 5,525				$ 5,533	$ 5,686
Related party transaction, annual rents due \| $								$ 207,664
Five Star \| Lease No. 1
Concentration Risk [Line Items]
Number of communities									82
Five Star \| Lease No. 2
Concentration Risk [Line Items]
Number of communities									47
Five Star \| Lease No. 3
Concentration Risk [Line Items]
Number of communities									17
Five Star \| Lease No. 4
Concentration Risk [Line Items]
Number of communities									29
Five Star \| Lease No. 5
Concentration Risk [Line Items]
Number of communities									9
Acquisitions \| Senior Living Communities
Concentration Risk [Line Items]
Number of units \| living_unit				709	709		366	366	366	366	366		709
Acquisitions \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Number of communities managed			5
Number of management agreements \| pooling_agreement			2
Number of communities owned and managed by related party acquired during the period \| property			6
Acquisitions \| Alabama and Indiana \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Number of communities owned and managed by related party acquired during the period \| property			2
Acquisitions \| Tennessee \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Number of communities owned and managed by related party acquired during the period \| property			1
Acquisitions \| Arizona \| Five Star \| Senior Living Communities
Concentration Risk [Line Items]
Number of communities owned and managed by related party acquired during the period \| property			1
Five Star
Concentration Risk [Line Items]
Minimum percentage of ownership of lessee's voting stock above which the entity has the option to cancel all its rights							9.80%	9.80%	9.80%	9.80%	9.80%