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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: