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Summary of Significant Accounting Policies (Details 2) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Investment in available for sale securities    
Fair value of the investments in available for sale securities $ 23,993us-gaap_AvailableForSaleSecurities $ 29,078us-gaap_AvailableForSaleSecurities
CommonWealth REIT (EQC)    
Investment in available for sale securities    
Investment in common shares 250,000us-gaap_InvestmentOwnedBalanceShares
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember		  
Weighted average quoted market prices (in dollars per share) $ 25.67snh_InvestmentWeightedAverageCostPerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember		 $ 26.00snh_InvestmentWeightedAverageCostPerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember
Fair value of the investments in available for sale securities 6,418us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember		 5,828us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember
Unrealized loss 83snh_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember		 673snh_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember
Five Star    
Investment in available for sale securities    
Investment in common shares 4,235,000us-gaap_InvestmentOwnedBalanceShares
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember		  
Percentage of common shares owned 8.60%snh_InvestmentOwnedPercentageOfTotalSharesOutstanding
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember		  
Weighted average quoted market prices (in dollars per share) $ 4.15snh_InvestmentWeightedAverageCostPerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember		 $ 3.36snh_InvestmentWeightedAverageCostPerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember
Fair value of the investments in available for sale securities 17,575us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember		 23,250us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember
Unrealized gain $ 3,361snh_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember		 $ 9,036snh_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember