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Significant Tenant (Details) (Five Star)
3 Months Ended
Mar. 31, 2015
Rents from significant lessee  
% of Total (as a percent) 32.70%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ dei_LegalEntityAxis
= snh_FiveStarQualityCareIncMember
Investment  
% of Total (as a percent) 31.80%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= snh_RealEstateInvestmentPropertyAtCostMember
/ dei_LegalEntityAxis
= snh_FiveStarQualityCareIncMember