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

This element represents the number of buildings sold by the entity.

+ References

No definition available.

+ Details

Name:	snh_NumberOfBuildingsSold
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 sold.

+ References

No definition available.

+ Details

Name:	snh_NumberOfCommunitiesSold
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 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 number of tenants.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of units within the community that was sold during the period.

+ References

No definition available.

+ Details

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

- Definition

Number of Real Estate Properties Sold

+ References

No definition available.

+ Details

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

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228

Reference 2: 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=98514028&loc=d3e3602-108585

+ Details

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

- Definition

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainLossOnSaleOfProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Carrying amount as of the balance sheet date of investments in land and buildings held for sale, excluding real estate considered to be inventory of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Details

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

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DiscontinuedOperationsHeldforsaleMember
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_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=snh_TripleNetSeniorLivingCommunitiesMember
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_LandParcelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_PA
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_RelatedPartyTransactionsByRelatedPartyAxis=snh_FiveStarQualityCareIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_IA
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_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Real Estate Properties - Impairment and Dispositions - (Details) $ in Thousands	1 Months Ended											3 Months Ended		5 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) living_unit property	Dec. 31, 2016 USD ($) property	Sep. 30, 2016 USD ($)	Jul. 31, 2016 USD ($) building property	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) living_unit	Aug. 31, 2015 USD ($) living_unit	Jul. 31, 2015 USD ($) living_unit	Apr. 30, 2015 USD ($) building property	Feb. 28, 2015 USD ($) living_unit	Mar. 31, 2018 USD ($) community	Mar. 31, 2016 community	Sep. 30, 2015 community	Dec. 31, 2017 USD ($) living_unit community property	Dec. 31, 2016 USD ($) living_unit property	Dec. 31, 2016 USD ($) building property	Dec. 31, 2016 USD ($) property	Dec. 31, 2016 USD ($) community property	Dec. 31, 2016 USD ($) property operator	Dec. 31, 2015 USD ($) living_unit community	Dec. 31, 2015 building
Real Estate Properties [Line Items]
Impairment of assets															$ 5,082			$ 18,674			$ 194
Number of communities \| community															305
Gain on sale of properties															$ 46,055			4,061			0
Increase (decrease) in annual lease rent payable															$ 4,870			$ 2,393
Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Impairment of assets																					$ 796
Discontinued operations, held for sale
Real Estate Properties [Line Items]
Number of real estate properties held for sale \| property	4	0													4	0	0	0	0	0
Number of units \| living_unit	1,295														1,295
Real estate held for sale	$ 53,338	$ 55,681													$ 53,338	$ 55,681	$ 55,681	$ 55,681	$ 55,681	$ 55,681
Senior Living Communities
Real Estate Properties [Line Items]
Number of communities \| community														19
Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Impairment of assets															0			11,488
Number of communities sold \| community																			2
Number of communities \| community																					3
Number of properties sold \| property	1
Sale price of property sold and agreed to be sold	$ 55,000
Gain on sale of properties	45,901
Triple Net Leased Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Impairment of assets																		4,391
Number of communities \| community																			2
Number of Tenants \| operator																				2
Triple Net Leased Senior Living Communities \| Discontinued operations, held for sale
Real Estate Properties [Line Items]
Number of communities \| community													2
MOBs \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Number of real estate properties sold																2	5
Number of real estate properties held for sale							1														1	4
Number of properties sold \| property				4						1
Sale price of property sold and agreed to be sold				$ 20,150						$ 1,500
Number of buildings sold \| building				4						4
Land Parcel \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Number of real estate properties sold \| living_unit																1
Sale price of property sold and agreed to be sold						$ 700
Land Parcel \| Eminent domain
Real Estate Properties [Line Items]
Gain on sale of properties	$ 154
Skilled Nursing Facility \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Sale price of property sold and agreed to be sold			$ 248		$ 9,100
Gain on sale of properties					$ 4,061
Pennsylvania \| MOBs \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Number of properties sold \| property		1
Sale price of property sold and agreed to be sold		$ 2,800
Florida \| MOBs \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Sale price of property sold and agreed to be sold		$ 2,100
Five Star
Real Estate Properties [Line Items]
Increase (decrease) in annual lease rent payable											$ 39				$ 3,193			$ 1,719			$ 1,734
Five Star \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Increase (decrease) in annual lease rent payable			25
Five Star \| Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Sale price of property sold and agreed to be sold							$ 21
Increase (decrease) in annual lease rent payable							$ (2)
Number of units within a community sold \| living_unit							117
Five Star \| Pennsylvania \| Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Sale price of property sold and agreed to be sold											250
Increase (decrease) in annual lease rent payable											$ (23)
Number of units within a community sold \| living_unit											120
Five Star \| Florida \| Senior Living Communities
Real Estate Properties [Line Items]
Increase (decrease) in annual lease rent payable			$ 10
Five Star \| Iowa \| Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Sale price of property sold and agreed to be sold									$ 155
Increase (decrease) in annual lease rent payable									$ (16)
Number of units within a community sold \| living_unit									12
Five Star \| Wisconsin \| Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Sale price of property sold and agreed to be sold								$ 850
Increase (decrease) in annual lease rent payable								$ (85)
Number of units within a community sold \| living_unit								63
Forecast \| Senior Living Communities \| Discontinued operations, disposed of by sale
Real Estate Properties [Line Items]
Number of properties sold \| community												3
Sale price of property sold and agreed to be sold												$ 313,000