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Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share award plans |  |  |
Share-based compensation expense | $ 1,914 us-gaap_AllocatedShareBasedCompensationExpense | $ 1,961 us-gaap_AllocatedShareBasedCompensationExpense | $ 1,827 us-gaap_AllocatedShareBasedCompensationExpense
Share Award Plans |  |  |
Share award plans |  |  |
Remaining common shares available for issuance | 2,725,233 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
Number of Shares |  |  |
Unvested shares at the beginning of the year | 150,711 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | 140,034 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | 132,620 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Shares granted | 94,200 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | 92,075 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | 87,092 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Shares vested | (91,433)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | (81,398)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | (79,678)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Unvested shares at the end of the year | 153,477 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | 150,711 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | 140,034 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Weighted Average Grant Date Fair Value |  |  |
Unvested shares at the beginning of the year (in dollars per share) | $ 23.84 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 23.03 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 22.86 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Shares granted (in dollars per share) | $ 21.83 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 23.48 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 22.38 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Shares vested (in dollars per share) | $ 21.87 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 23.85 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 22.18 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Unvested shares at the end of the year (in dollars per share) | $ 23.39 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 23.84 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember | $ 23.03 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember
Scheduled to vest unvested shares |  |  |
2015 | 59,133 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestInNextFiscalYear / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
2016 | 46,193 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestInTwoYears / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
2017 | 31,810 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestInThreeYears / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
2018 | 16,340 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestInFourYears / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
Estimated future compensation for the unvested shares | 3,393 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
Closing share price (in dollars per share) | $ 22.11 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedClosingPrice / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember |  |
Weighted average period over which the compensation expense will be recorded | 1 year 9 months 18 days |  |
Share Award Plans | Trustees |  |  |
Share award plans |  |  |
Common shares awarded | 2,500 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember | 2,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember | 2,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember
Aggregate market value of shares awarded | 306 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateMarketValueOfSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember | 286 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateMarketValueOfSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember | 205 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateMarketValueOfSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember
Market value of shares awarded to each Trustee | 61 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketValueOfSharesIssuedInPeriodToEachIndividual / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember | 57 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketValueOfSharesIssuedInPeriodToEachIndividual / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember | 41 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketValueOfSharesIssuedInPeriodToEachIndividual / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_TitleOfIndividualAxis = snh_TrusteesMember
Share Award Plans | RMR | Officers and Employees |  |  |
Share award plans |  |  |
Common shares awarded | 81,700 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = snh_ReitManagementAndResearchLLCMember / us-gaap_TitleOfIndividualAxis = snh_OfficersAndEmployeesMember | 82,600 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = snh_ReitManagementAndResearchLLCMember / us-gaap_TitleOfIndividualAxis = snh_OfficersAndEmployeesMember | 78,492 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = snh_ReitManagementAndResearchLLCMember / us-gaap_TitleOfIndividualAxis = snh_OfficersAndEmployeesMember
Aggregate market value of shares awarded | $ 1,750 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateMarketValueOfSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = snh_ReitManagementAndResearchLLCMember / us-gaap_TitleOfIndividualAxis = snh_OfficersAndEmployeesMember | $ 1,888 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateMarketValueOfSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = snh_ReitManagementAndResearchLLCMember / us-gaap_TitleOfIndividualAxis = snh_OfficersAndEmployeesMember | $ 1,775 snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateMarketValueOfSharesIssuedInPeriod / us-gaap_PlanNameAxis = snh_ShareAwardPlansMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = snh_ReitManagementAndResearchLLCMember / us-gaap_TitleOfIndividualAxis = snh_OfficersAndEmployeesMember
Award vesting period | 5 years |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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