Basis of Presentation (Details) $ in Thousands	6 Months Ended
Jun. 30, 2019 USD ($) building property	Dec. 31, 2018 USD ($)
Basis Of Presentation [Line Items]
Variable interest entity, consolidated, carrying amount, assets	$ 1,036,537	$ 1,061,593
Variable interest entity, consolidated, carrying amount, liabilities	$ (707,579)	$ (714,226)
Joint Venture
Basis Of Presentation [Line Items]
Noncontrolling interest, ownership percentage by parent	45.00%
Ownership percentage	55.00%
Massachusetts \| MOBs \| Joint Venture
Basis Of Presentation [Line Items]
Number of properties included in joint venture agreement \| property	1
Number of buildings included in joint venture agreement \| building	2

[Line Items] for Basis Of Presentation [Table]

No definition available.

Name:	snh_BasisOfPresentationLineItems
Namespace Prefix:	snh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of Buildings Included in Joint Venture Agreement

+ References

No definition available.

+ Details

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

- Definition

Number of Properties Included in Joint Venture Agreement

+ References

No definition available.

+ Details

Name:	snh_NumberofPropertiesIncludedinJointVentureAgreement
Namespace Prefix:	snh_
Data Type:	xbrli:integerItemType
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

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

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

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

- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5710-111685

+ Details

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

- Definition

The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.

+ References

+ Details

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

- Details

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

- Details

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

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=snh_MedicalOfficeBuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: