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Related Person Transactions - Narrative (Details) $ / shares in Units, $ in Thousands |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
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Nov. 09, 2017 USD ($) property | Nov. 10, 2016 $ / shares shares | Dec. 14, 2015 shares | Jun. 05, 2015 USD ($) shares | Jan. 29, 2015 USD ($) property | Mar. 31, 2017 building | Sep. 30, 2015 USD ($) | May 31, 2015 USD ($) | Apr. 30, 2015 USD ($) property | Jan. 31, 2015 USD ($) living_unit | Dec. 31, 2015 USD ($) community | Dec. 31, 2017 USD ($) community building property party lease_agreement trustee agreement shares | Dec. 31, 2016 USD ($) community property shares | Dec. 31, 2015 USD ($) community property shares | Dec. 31, 2001
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on distribution to common shareholders of The RMR Group Inc. common stock |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 38,437 |
Number Of Independent Trustees | trustee |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |
Fair value adjustment, liabilities |  |  |  |  |  |  |  |  |  |  |  | $ 1,906 |  |  |
Equity in earnings of an investee |  |  |  |  |  |  |  |  |  |  |  | 608 | 137 | 20 |
Other comprehensive income (loss) attributable to equity method investee, net of tax |  |  |  |  |  |  |  |  |  |  |  | $ (462) | (152) | 20 |
Number of buildings included in joint venture agreement | building |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
The RMR Group Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in common shares | shares |  |  |  |  |  |  |  |  |  |  |  | 2,637,408 |  |  |
Economic interest percent |  |  |  |  |  |  |  |  |  |  |  | 51.90% |  |  |
Voting power percent |  |  |  |  |  |  |  |  |  |  |  | 91.40% |  |  |
Number Of Unrelated Parties | party |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |
Related party transaction, rate |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |
RMR Inc. | Class A common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on distribution to common shareholders of The RMR Group Inc. common stock |  |  |  |  |  |  |  |  |  |  | $ 38,437 |  |  |  |
RMR Inc. | Up C Transaction |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash purchase price |  |  |  | $ 13,967 |  |  |  |  |  |  |  |  |  |  |
RMR Inc. | Up C Transaction | Common Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued in connection with acquisition | shares |  |  |  | 2,345,000 |  |  |  |  |  |  |  |  |  |  |
RMR LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rental revenue from related party |  |  |  |  |  |  |  |  |  |  |  | $ 331 | $ 242 | $ 204 |
RMR Inc. | Class A common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares distributed to stockholders | shares |  |  | 2,635,379 |  |  |  |  |  |  |  |  |  |  |  |
RMR Inc. | Up C Transaction | Managing Trustees Barry And Adam Portnoy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares under lock up and registration agreement | shares |  |  |  | 2,345,000 |  |  |  |  |  |  |  |  |  |  |
Period over which shares remain under lock up and registration agreement |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |
RMR Inc. | RMR Inc. | Up C Transaction | Class B2 common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued in connection with acquisition | shares |  |  |  | 5,272,787 |  |  |  |  |  |  |  |  |  |  |
Senior Living Communities | Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties acquired or agreed to be acquired | property |  |  |  |  |  |  |  |  |  |  |  |  | 10 | 39 |
Purchase price excluding closing costs and working capital adjustments |  |  |  |  |  |  | $ 18,250 | $ 9,750 |  |  |  |  |  |  |
MOBs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Standalone Insurance Policy Coverage Period |  |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |
Number of buildings included in joint venture agreement | building |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
MOBs | Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties acquired or agreed to be acquired |  |  |  |  |  |  |  |  |  | 23 |  | 6 | 3 | 23 |
Purchase price excluding closing costs and working capital adjustments |  |  |  |  |  |  |  |  |  | $ 539,000 |  |  |  |  |
MOBs | SIR | Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt disposed of from properties sold |  |  |  |  | $ 29,995 |  |  |  |  |  |  |  |  |  |
Amount from SIR related to working capital activity |  |  |  |  |  |  |  |  | $ 1,316 |  |  |  |  |  |
Five Star |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total shares outstanding |  |  |  |  |  |  |  |  |  |  |  | 8.40% |  |  | 100.00%
Investment in common shares | shares |  |  |  |  |  |  |  |  |  |  |  | 4,235,000 |  |  |
Maximum percentage of any class of equity shares that can be acquired without approval |  |  |  |  |  |  |  |  |  |  |  | 9.80% |  |  |
Minimum percentage of ownership of lessee's voting stock above which the entity has the option to cancel all its rights |  |  |  |  |  |  |  |  |  |  |  | 9.80% |  |  |
Number of real estate properties leased | community |  |  |  |  |  |  |  |  |  |  | 177 | 185 | 185 | 177 |
Property subject to or available for operating lease, number of leases | lease_agreement |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |
Related party transactions, shares authorized to purchase | shares |  | 18,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Related party transaction, percent of shares issued and outstanding |  | 38.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Related party transaction, shares purchased | shares |  | 17,999,999 |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) | $ / shares |  | $ 3.00 |  |  |  |  |  |  |  |  |  |  |  |  |
Five Star | RMR LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total shares outstanding |  |  |  |  |  |  |  |  |  |  |  | 36.40% |  |  |
Investment in common shares | shares |  |  |  |  |  |  |  |  |  |  |  | 18,382,121 |  |  |
Five Star | Senior Living Communities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of communities managed | community |  |  |  |  |  |  |  |  |  |  |  | 70 | 68 | 60 |
Five Star | Senior Living Communities | Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of communities managed | community |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |
Number of properties acquired or agreed to be acquired | property | 6 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price excluding closing costs and working capital adjustments | $ 104,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
RMR LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of management agreements with related party | agreement |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
Shares issued in connection with acquisition | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 81,557 |
Premium payable for combined directors' and officers' liability insurance policy |  |  |  |  |  |  |  |  |  |  | $ 472 | $ 255 | $ 142 | $ 472 |
RMR LLC | Restricted shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares granted under the award plan | shares |  |  |  |  |  |  |  |  |  |  |  | 88,100 | 79,650 | 84,000 |
Aggregate value of awards granted during the period |  |  |  |  |  |  |  |  |  |  |  | $ 1,743 | $ 1,724 | $ 1,321 |
SIR | MOBs | Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties acquired or agreed to be acquired | property |  |  |  |  | 23 |  |  |  | 23 |  |  |  |  |  |
Purchase price excluding closing costs and working capital adjustments |  |  |  |  | $ 532,000 |  |  |  |  |  |  |  |  |  |
Fair value adjustment, liabilities |  |  |  |  | $ 7,677 |  |  |  |  |  |  |  |  |  |
AIC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total premiums including taxes and fees |  |  |  |  |  |  |  |  |  |  |  | 2,433 | 3,607 | $ 2,785 |
Equity method investments, carrying value |  |  |  |  |  |  |  |  |  |  |  | $ 8,185 | $ 7,116 |  |
Year one | RMR LLC | RMR Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |
Year two | RMR LLC | RMR Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |
Year three | RMR LLC | RMR Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |
Year four | RMR LLC | RMR Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |
Year five | RMR LLC | RMR Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |

X
- Definition Represent the amounts due from related person, owed to us from SIR related to working capital activity for MOBs. + References No definition available. + Details
Name: | snh_AmountOwedFromSir
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the amount of premium payable for combined directors' and officers' liability insurance policy. + References No definition available. + Details
Name: | snh_CombinedDirectorsAndOfficersLiabilityInsurancePolicyPremiumPayable
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Represents the percentage of economic interest held in RMR Inc. + References No definition available. + Details
Name: | snh_EconomicInterestPercent
Namespace Prefix: | snh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition This element represents the amount of annual premiums including taxes and fees for property insurance pursuant to an insurance program arranged by the equity method investee. + References No definition available. + Details
Name: | snh_EquityMethodInvestmentPropertyInsuranceAnnualPremiumAmount
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the number of shares held as a percentage of the total shares outstanding at the end of the period. + References No definition available. + Details
Name: | snh_InvestmentOwnedPercentageOfTotalSharesOutstanding
Namespace Prefix: | snh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of the company's common shares under a lock up and registration rights agreement that are not authorized to be transferred for a specified period of time. + References No definition available. + Details
Name: | snh_LockUpAgreementTransferOfCommonShares
Namespace Prefix: | snh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the period over which the company's common shares remain under a lock up and registration rights agreement and not authorized to be transfered. + References No definition available. + Details
Name: | snh_LockUpAgreementTransferOfCommonSharesPeriod
Namespace Prefix: | snh_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the minimum percentage of common stock held in the investee above which the entity would have the option to cancel all its rights. + References No definition available. + Details
Name: | snh_MinimumInvestmentOwnershipPercentageEligibleForOptionToCancelRightsOfLessee
Namespace Prefix: | snh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of non cash loss, which represents difference between our carrying value and the fair value of the RMR Inc. shares on the distribution date + References No definition available. + Details
Name: | snh_NonCashLossOnSpecialDistributionOfEquity
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the number of communities managed by a related party. + References No definition available. + Details
Name: | snh_NumberOfCommunitiesManagedByRelatedParty
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Number Of Independent Trustees + References No definition available. + Details
Name: | snh_NumberOfIndependentTrustees
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents the number of properties acquired or agreed to be acquired by the entity. + References No definition available. + Details
Name: | snh_NumberOfPropertiesAcquired
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Number Of Unrelated Parties + References No definition available. + Details
Name: | snh_NumberOfUnrelatedParties
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of Buildings Included in Joint Venture Agreement + References No definition available. + Details
Name: | snh_NumberofBuildingsIncludedinJointVentureAgreement
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the increase (decrease) in the entity's share of other comprehensive income of the investee. + References No definition available. + Details
Name: | snh_OtherComprehensiveIncomeLossAttributableToEquityMethodInvesteeNetOfTax
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Property Subject to or Available for Operating Lease, Number of Leases + References No definition available. + Details
Name: | snh_PropertySubjecttoorAvailableforOperatingLeaseNumberofLeases
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the aggregate purchase price of real estate properties acquired by the entity during the period. This amount includes cash paid plus debt assumed, if any, and other settlement adjustments but excludes closing costs. + References No definition available. + Details
Name: | snh_RealEstateAggregatePurchasePriceIncludingOtherSettlementAdjustments
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the debt disposed of in sale of property transactions. + References No definition available. + Details
Name: | snh_RealEstateDebtDisposedOfInSaleOfPropertyTransactions
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Related Party, Management Agreements + References No definition available. + Details
Name: | snh_RelatedPartyManagementAgreements
Namespace Prefix: | snh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents the cash purchase price paid to acquire common shares of beneficial interest of related party pursuant to the stock purchase agreement. + References No definition available. + Details
Name: | snh_RelatedPartyTransactionCashPurchasePrice
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Related Party Transaction, Percent of Shares Issued and Outstanding + References No definition available. + Details
Name: | snh_RelatedPartyTransactionPercentofSharesIssuedandOutstanding
Namespace Prefix: | snh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents the number of shares purchased, from related party. + References No definition available. + Details
Name: | snh_RelatedPartyTransactionSharesPurchased
Namespace Prefix: | snh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Related Party Transactions, Shares Authorized to Purchase + References No definition available. + Details
Name: | snh_RelatedPartyTransactionsSharesAuthorizedtoPurchase
Namespace Prefix: | snh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the aggregate fair value of the equity instruments other than options granted during the period. + References No definition available. + Details
Name: | snh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
Namespace Prefix: | snh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the maximum percentage of the equity shares of the entity which any single person or group can acquire without obtaining approval. + References No definition available. + Details
Name: | snh_ShareOwnershipRestrictionsMaximumPercentageOfEquitySharesThatCanBeAcquiredWithoutApproval
Namespace Prefix: | snh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of shares distributed to the shareholders. + References No definition available. + Details
Name: | snh_SharesDistributedToStockholders
Namespace Prefix: | snh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Standalone Insurance Policy Coverage Period + References No definition available. + Details
Name: | snh_StandaloneInsurancePolicyCoveragePeriod
Namespace Prefix: | snh_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the percentage of voting power controlled by a given entity with outstanding capital stock in RMR Inc. + References No definition available. + Details
Name: | snh_VotingPowerPercent
Namespace Prefix: | snh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of shares of equity interests issued or issuable to acquire entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_EquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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