Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2015
Summary of Significant Accounting Policies
BASIS OF PRESENTATION	BASIS OF PRESENTATION. Our consolidated financial statements include the accounts of Senior Housing Properties Trust, or SNH, we, us or our, and our subsidiaries, all of which are 100% owned directly or indirectly by us. All intercompany transactions and balances with or among our consolidated subsidiaries have been eliminated. Accounting principles generally accepted in the United States, or GAAP, require us to make estimates and assumptions that may affect the amounts reported in these financial statements and related notes. The actual results could differ from these estimates. We have made reclassifications to the prior years’ financial statements to conform to the current year’s presentation. These reclassifications had no effect on net income or shareholders’ equity. We have determined that, in 2015, certain of our wholly owned taxable REIT subsidiaries, or TRSs, were variable interest entities, or VIEs, as defined under the Consolidation Topic of the Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or the Codification. We have concluded that we must consolidate each of these TRSs because we are the entity with the power to direct the activities that most significantly impact such VIEs’ performance and we have the obligation to absorb losses or the right to receive benefits from each VIE that could be significant to the VIE and are, therefore, the primary beneficiary of each VIE. The assets of our TRSs classified as VIEs were $2,879 as of December 31, 2015 and consist primarily of working capital of one of our senior living managers. The liabilities of our TRSs classified as VIEs were $950 as of December 31, 2015 and consist primarily of payables to one of our senior living managers. The assets of our TRSs are available to satisfy our TRSs’ obligations and we have guaranteed certain obligations of our TRSs.
REAL ESTATE PROPERTIES	REAL ESTATE PROPERTIES. We record properties at our cost and calculate depreciation on real estate investments on a straight line basis over estimated useful lives generally up to 40 years. When we acquire a property, we estimate the purchase price allocations and the useful lives of our properties. In some circumstances, we engage third party real estate appraisal firms to provide market information and evaluations which are relevant to our purchase price allocations and determinations of useful lives; however, we are ultimately responsible for the purchase price allocations and determinations of useful lives. We allocate the purchase prices of our properties to land, building and improvements based on determinations of the fair values of these assets assuming the properties are vacant. We determine the fair value of each property using methods similar to those used by third party appraisers. For properties qualifying as acquired businesses under Accounting Standards Codification 805, Business Combinations, we allocate a portion of the purchase price of our properties to above market and below market leases based on the present value (using an interest rate which reflects the risks associated with acquired in place leases at the time each property was acquired by us) of the difference, if any, between (i) the contractual amounts to be paid pursuant to the acquired in place leases and (ii) our estimates of fair market lease rates for the corresponding leases, measured over a period equal to the terms of the respective leases. We allocate a portion of the purchase price to acquired in place leases and tenant relationships based upon market estimates to lease up the property based on the leases in place at the time of purchase. We amortize capitalized above market lease values (included in acquired real estate leases in our consolidated balance sheets) as a reduction to rental income over the remaining non‑cancelable terms of the respective leases. We amortize capitalized below market lease values (presented as acquired real estate lease obligations in our consolidated balance sheets) as an increase to rental income over the non‑cancelable periods of the respective leases. Such amortization resulted in an increase in rental income of $4,060 during the year ended December 31, 2015, an increase in rental income of $2,322 during the year ended December 31, 2014, and a reduction in rental income of $3,656 during the year ended December 31, 2013. We amortize the value of in place leases exclusive of the value of above market and below market in place leases to expense over the remaining non‑cancelable periods of the respective leases. Such amortization included in depreciation totaled $80,040, $38,970 and $22,718, during the years ended December 31, 2015, 2014 and 2013, respectively. If a lease is terminated prior to its stated expiration, the unamortized amount relating to that lease is written off.
CASH AND CASH EQUIVALENTS	CASH AND CASH EQUIVALENTS. We carry cash and cash equivalents, consisting of overnight repurchase agreements and short term investments with original maturities of three months or less at the date of purchase, at cost plus accrued interest, which approximates fair value.
RESTRICTED CASH	RESTRICTED CASH. Restricted cash consists of amounts escrowed for real estate taxes, insurance and capital expenditures at certain of our mortgaged properties and security deposits for residents of our managed senior living communities.
INVESTMENTS IN AVAILABLE FOR SALE SECURITIES	INVESTMENTS IN AVAILABLE FOR SALE SECURITIES. At December 31, 2014, we owned 250,000 common shares of Equity Commonwealth (f/k/a CommonWealth REIT), or EQC. We sold all 250,000 of our EQC common shares in May 2015 for net proceeds of $6,571. We recognized a gain on the sale of these shares in the amount of $71 during the second quarter of 2015, which is included in interest and other income in our consolidated statements of comprehensive income. We owned 4,235,000 common shares, or 8.6% at December 31, 2015 and 2014, of Five Star Quality Care, Inc., or Five Star. We classify these shares of Five Star as available for sale securities and carry them at fair value, with unrealized gains and losses reported as a component of shareholders’ equity. Cumulative other comprehensive income shown in our consolidated balance sheets includes the net unrealized gain or loss on investments determined as the net difference between the market value of these shares of Five Star calculated by using weighted average quoted market prices on the dates we acquired these shares ($3.36 per share) and on December 31, 2015($3.18 per share). At December 31, 2015 and 2014, our investment in Five Star had a fair value of $13,467 and $17,575, respectively, including an unrealized loss of $747 and an unrealized gain of $3,361, respectively. In addition, at December 31, 2015, we owned 2,637,408 shares of class A common stock of The RMR Group Inc., or RMR Inc. We also classify these shares of RMR Inc. as available for sale securities and carry them at fair value, with unrealized gains and losses reported as a component of shareholders’ equity. Our historical cost basis for these shares is $69,826. At December 31, 2015, our investment in RMR Inc. had a fair value of $38,005, including an unrealized loss of $31,821 based on RMR Inc.’s quoted share price at December 31, 2015 ($14.41 per share). We evaluate our investments in available for sale securities to determine if a decline in the fair value below our carrying value is other than temporary. We consider the severity and the duration of the decline, and our ability and intent to hold the investment until recovery when making this assessment. If a decline in fair value is determined to be other than temporary, an impairment loss equal to the difference between the investment’s carrying value and its fair value is recognized in earnings. See Notes 5 and 7 for further information regarding our investments in Five Star and RMR Inc.
EQUITY METHOD INVESTMENTS	EQUITY METHOD INVESTMENTS. At December 31, 2015, we owned 14.3% of Affiliates Insurance Company, or AIC’s, outstanding equity. Although we own less than 20% of AIC, we use the equity method to account for this investment because we believe that we have significant influence over AIC because all of our Trustees are also directors of AIC. Under the equity method, we record our percentage share of net earnings from AIC in our consolidated statements of comprehensive income. If we determine there is an “other than temporary impairment” in the fair value of this investment, we would record a charge to earnings. In evaluating the fair value of this investment, we have considered, among other things, the assets and liabilities held by AIC, AIC’s overall financial condition and earning trends, and the financial condition and prospects for the insurance industry generally. See Note 5 for further information regarding our investment in AIC.
DEFERRED FINANCING FEES	DEFERRED FINANCING FEES. We capitalize issuance costs related to borrowings and amortize them over the terms of the respective borrowings. During 2015, we capitalized $2,952 of issuance costs, including $1,200 related to an amendment to our revolving credit facility agreement to increase the maximum borrowings under that facility in September 2015, as well as $1,752 related to our $200,000 term loan entered into in September 2015. During 2014, we capitalized $8,039 of issuance costs, including $5,021 related to our issuances of senior unsecured notes in April 2014, as well as $2,819 related to our $350,000 term loan entered into in May 2014, and $199 related to our assumption of a mortgage in May 2014. During 2013, we capitalized $3,326 of issuance costs, including $3,078 related to an amendment to our revolving credit facility agreement in September 2013 and $248 related to our assumption of a mortgage during 2013. During 2015, we wrote off $70 of unamortized deferred financing fees in connection with our prepayment of the outstanding principal balances of ten mortgages encumbering nine properties with an aggregate principal balance of $82,569. We also wrote off $74 of unamortized deferred financing fees in connection with our prepayment of all $250,000 of our 4.30% senior unsecured notes due January 2016 in November 2015. During 2014, we wrote off $357 of unamortized deferred financing fees in connection with our prepayment of the outstanding principal balances of two mortgages totaling $23,234 in October and December 2014. The unamortized gross balance of deferred financing fees and related accumulated amortization was $50,477 and $22,782, and $50,479 and $19,930, at December 31, 2015 and 2014, respectively. At December 31, 2015, the remaining weighted average amortization period is approximately 12.5 years. We expect that the amortization expense relating to the unamortized gross balance of deferred financing fees for the five years subsequent to December 31, 2015 will be $5,514 in 2016, $5,247 in 2017, $3,394 in 2018, $2,639 in 2019, $1,213 in 2020, and $9,688, thereafter.
DEFERRED LEASING COSTS	DEFERRED LEASING COSTS. Deferred leasing costs include brokerage, legal and other fees associated with our entering leases and are amortized on a straight line basis over the terms of the respective leases. Deferred leasing costs are included in other assets on our consolidated balance sheets. The unamortized gross balance of deferred leasing costs and related accumulated amortization was $21,708 and $5,561, and $14,844 and $3,722 at December 31, 2015 and 2014, respectively. At December 31, 2015, the remaining weighted average amortization period is approximately 6.8 years. We expect that the amortization expense for the five years subsequent to December 31, 2015 will be $2,853 in 2016, $2,528 in 2017, $2,286 in 2018, $1,968 in 2019, $1,583 in 2020, and $4,710, thereafter.
REVENUE RECOGNITION	REVENUE RECOGNITION. We recognize rental income from operating leases on a straight line basis over the term of each lease agreement. We recognize percentage rents when realizable and earned, which is generally during the fourth quarter of the year. For the years ended December 31, 2015, 2014 and 2013, percentage rents earned aggregated $10,062, $10,155 and $9,226, respectively. As of December 31, 2015, we owned 65 managed senior living communities, including 55 communities that we acquired since June 2011 and ten senior living communities formerly leased to Sunrise Senior Living, Inc. Sixty of these communities are managed by Five Star, and the remaining five are managed by a private senior living manager. We refer to these 65 communities as the managed senior living communities. We derive our revenues at these managed senior living communities primarily from services our managers provide to residents on our behalf and we record revenues when services are provided. We use the TRS structure authorized by the Real Estate Investment Trust Investment Diversification and Empowerment Act for nearly all of our managed senior living communities.
EARNINGS PER COMMON SHARE	PER COMMON SHARE AMOUNTS. We calculate basic earnings per common share by dividing net income by the weighted average number of our common shares of beneficial ownership, $0.01 par value, or our common shares, outstanding during the period. We calculate diluted earnings per common share, or EPS, using the more dilutive of the two class method or the treasury stock method. Unvested share awards and other potentially dilutive common shares and the related impact on earnings, are considered when calculating diluted earnings per share.
INCOME TAXES	INCOME TAXES. We have elected to be taxed as a REIT under the Internal Revenue Code of 1986, as amended, and as such are generally not subject to federal and most state income taxation on our operating income, provided we distribute our taxable income to our shareholders and meet certain organization and operating requirements. We do, however, lease nearly all of our managed senior living communities to our TRSs, that, unlike most of our subsidiaries, file separate tax returns and are subject to federal and state income taxes. Our consolidated income tax provision includes the income tax provision related to the operations of our TRSs and certain state income taxes incurred by us, despite our REIT status. The Income Taxes Topic of the Codification prescribes how we should recognize, measure and present in our financial statements uncertain tax positions that have been taken or are expected to be taken in a tax return. Tax benefits are recognized to the extent that it is “more likely than not” that a particular tax position will be sustained upon examination or audit. To the extent the “more likely than not” standard has been satisfied, the benefit associated with a tax position is measured as the largest amount that has a greater than 50% likelihood of being realized upon settlement. We classify interest and penalties related to uncertain tax positions, if any, in our financial statements as a component of general and administrative expense.
SEGMENT REPORTING	SEGMENT REPORTING. As of December 31, 2015, we have four operating segments, of which three are separate reporting segments. The first reporting segment includes triple net senior living communities that provide short term and long term residential care and other services for residents. The second reporting segment includes managed senior living communities that provide short term and long term residential care and other services for residents. The third reporting segment includes properties leased to medical providers, medical related businesses, clinics and biotech laboratory tenants, or MOBs. Our fourth segment includes the remainder of our operations, including certain properties that offer fitness, wellness and spa services to members, which we do not consider to be sufficiently material as to constitute a separate reporting segment, and all of our other operations. See Note 9 for further information regarding our reportable operating segments.
NEW ACCOUNTING PRONOUNCEMENTS	NEW ACCOUNTING PRONOUNCEMENTS. In February 2015, the FASB, issued Accounting Standards Update, or ASU, No. 2015-02, Consolidation. Among other things, this update changes how an entity determines the primary beneficiary of a VIE. This update is effective for interim and annual periods beginning after December 15, 2015, with early adoption permitted. The implementation of this update is not expected to cause any significant changes to our consolidated financial statements. In April 2015, the FASB issued ASU No. 2015-03, Simplifying the Presentation of Debt Issuance Costs, which requires debt issuance costs to be presented in the balance sheet as a direct deduction from the associated debt liability. In August 2015, the FASB clarified the previous ASU and issued ASU No. 2015-15, Presentation and Subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements – Amendments to SEC Paragraphs Pursuant to Staff Announcement at June 18, 2015 EITF Meeting, which addresses the presentation of debt issuance costs related to line of credit arrangements. These updates are effective for interim and annual reporting periods beginning after December 15, 2015 and require retrospective application. The implementation of these updates is not expected to cause any material changes to our consolidated financial statements other than the reclassification of debt issuance costs from assets to contra liabilities on our consolidated balance sheets. Debt issuance costs related to our revolving credit facility will remain classified as assets in accordance with ASU 2015-15. When these updates are adopted, the unamortized gross balance of deferred financing fees of $23,950 and $26,339 as of December 31, 2015 and 2014, respectively, will be reclassified from assets to the related debt obligations on our consolidated balance sheets. In May 2014, the FASB issued ASU No. 2014-09, Revenue From Contracts With Customers, which outlines a comprehensive model for entities to use in accounting for revenue arising from contracts with customers. This ASU states that “an entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services.” While this ASU specifically references contracts with customers, it may apply to certain other transactions such as the sale of real estate or equipment. In July 2015, the FASB approved a one year deferral of the effective date for this ASU to interim and annual reporting periods beginning after December 15, 2017. We are continuing to evaluate this guidance; however, we do not expect its adoption to have a material impact on our consolidated financial statements, as a substantial portion of our revenue consists of rental income from leasing arrangements, which are specifically excluded from this ASU. In September 2015, the FASB issued ASU No. 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, which eliminates the requirement for an acquirer in a business combination to account for measurement-period adjustments retrospectively. Instead, acquirers must recognize measurement-period adjustments during the period in which they determine the amounts, including the effect on earnings of any amounts they would have recorded in previous periods if the accounting had been completed at the acquisition date. This update is effective for interim and annual periods beginning after December 15, 2015, with early adoption permitted. The implementation of this update is not expected to cause any significant changes to our consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities, which changes how entities measure certain equity investments and present changes in the fair value of financial liabilities measured under the fair value option that are attributable to their own credit. This update is effective for interim and annual periods beginning after December 15, 2017, with early adoption permitted subject to certain conditions. We are continuing to evaluate this guidance; however, we expect the implementation of this guidance will change our accounting for our available for sale equity investments. Currently, changes in fair value of these investments are recorded through other comprehensive income. Under this ASU, these changes will be recorded through earnings.