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Unrealized Gain / Loss on Investments (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
CommonWealth REIT (EQC)  
Unrealized gain on investments  
Investment in common shares 250,000us-gaap_InvestmentOwnedBalanceShares
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember
Quoted market prices (in dollars per share) $ 26.55snh_InvestmentQuotedMarketPricePerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember
Weighted average costs (in dollars per share) $ 26.00snh_InvestmentWeightedAverageCostPerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_CommonWealthREITMember
Five Star  
Unrealized gain on investments  
Investment in common shares 4,235,000us-gaap_InvestmentOwnedBalanceShares
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember
Quoted market prices (in dollars per share) $ 4.44snh_InvestmentQuotedMarketPricePerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember
Weighted average costs (in dollars per share) $ 3.36snh_InvestmentWeightedAverageCostPerShare
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= snh_FiveStarQualityCareIncMember