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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.

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Face amount or stated value per share of common stock.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made 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).

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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).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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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.

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Net number of share options (or share units) granted during the period.

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Price of a single share of a number of saleable stocks of a company.

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Percentage of vesting of share-based compensation awards.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP), net of any shares forfeited. Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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Share-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions							12 Months Ended							14 Months Ended
	Jun. 15, 2016 USD ($) trustee $ / shares shares	Jan. 26, 2016 tranche $ / shares shares	Jun. 16, 2015 USD ($) trustee $ / shares shares	Jan. 28, 2015 $ / shares shares	Mar. 25, 2014 shares	Jan. 28, 2014 trustee $ / shares shares	Dec. 31, 2020 shares	Dec. 31, 2019 shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Jul. 31, 2014 trustee	Dec. 31, 2013 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)													4,000
Granted (in dollars per share) \| $ / shares											$ 0.00		$ 23.46
Number of independent trustees \| trustee														5
Unvested (in shares)													0		130,914
Vested (in shares)													134,914
Common shares of beneficial interest, par value (in dollars per share) \| $ / shares											$ 0.01	$ 0.01
Trustees \| Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares granted in period (in shares)						2,000
Common shares of beneficial interest, par value (in dollars per share) \| $ / shares						$ 0.01
Price per common share (in dollars per share) \| $ / shares						$ 23.46
Number of trustees \| trustee						2
Officers and Employees \| Employee Stock \| RMR
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense \| $													$ 3.4
Officers and Employees \| Employee Stock \| Common Shares \| RMR
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares issued (in shares)					130,914
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of incentive plan after effective date											4 years	4 years	4 years
Number of tranches \| tranche		2
Number of equity awards granted (in shares)		277,386		256,467
Unvested (in shares)											2,572,768	1,884,110	1,266,237		0
Vested (in shares)											0	0	0
Estimated future compensation expense for unvested shares \| $											$ 19.2
Weighted average period over which compensation expense will be recorded											2 years
Restricted stock units \| Vesting on January 26, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percent		50.00%
Restricted stock units \| Vesting on January 26, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percent		50.00%
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of independent trustees \| trustee	9		9
Share based compensation amount \| $	$ 0.1		$ 0.1
Number of equity awards granted (in shares)	31,167	136,623	34,587	126,319							167,790	160,906	713,727
Price per share (in dollars per share) \| $ / shares	$ 28.88		$ 26.02
Vesting period	1 year	4 years	1 year	4 years
Granted (in dollars per share) \| $ / shares		$ 26.93		$ 26.58							$ 27.29	$ 26.46	$ 26.66
Unvested (in shares)											752,209	776,456	710,182		0
Vested (in shares)											191,498	86,517	3,545
Estimated future compensation expense for unvested shares \| $											$ 12.4
Weighted average period over which compensation expense will be recorded											2 years 1 month 6 days
Restricted shares \| Scenario, Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)							67,029	95,554	373,140	216,486
Restricted shares \| Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)	3,463		3,843
Restricted stock units, maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		691,385		639,250									1,266,237
Granted (in dollars per share) \| $ / shares											$ 15.57	$ 15.47	$ 15.99
2015 Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of incentive plan after effective date			10 years
2015 Incentive Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares available for issuance (in shares)			3,250,000
Share award plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares available for issuance (in shares)											2,358,464
Share award plan \| General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense \| $											$ 18.5	$ 15.4	$ 2.6