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Real Estate Properties - Investments and Capital Expenditures - (Details) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 28, 2015 USD ($) | Dec. 31, 2017 USD ($) ft² | Dec. 31, 2016 USD ($) ft² | Dec. 31, 2015 USD ($)
Real Estate [Line Items] |  |  |  |
Real estate improvements by lessee purchased |  | $ 51,952 | $ 30,328 |
Increase (decrease) in annual lease rent payable |  | 4,870 | 2,393 |
Leases committed expenditure |  | $ 22,540 | $ 12,422 |
Area of real estate properties leased (in square feet) | ft² |  | 1,300,000 | 899,000 |
Lease committed but unspent tenant related obligations |  | $ 20,681 | $ 23,271 |
Five Star |  |  |  |
Real Estate [Line Items] |  |  |  |
Real estate improvements by lessee purchased |  | 39,800 | 21,438 | $ 21,444
Increase (decrease) in annual lease rent payable | $ 39 | $ 3,193 | $ 1,719 | $ 1,734

X
- 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

X
- Definition Amount of committed but unspent tenant related obligations based on executed lease. + References No definition available. + Details
Name: | snh_LeaseCommittedButUnspentTenantRelatedObligations
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the amount of expenditure committed by entity relating to leases. + References No definition available. + Details
Name: | snh_LeasesCommittedExpenditure
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- Definition Represents the area of the real estate property subject to an operating lease. + References No definition available. + Details
Name: | snh_SquareFootageOfRealEstatePropertyLeased
Namespace Prefix: | snh_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | duration

X
- 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_RealEstateLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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