Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share award plans
Share-based compensation expense	$ 1,827	$ 1,659	$ 1,445
Share Award Plans
Share award plans
Remaining common shares available for issuance	2,911,508
Number of Shares
Unvested shares at the beginning of the year	132,620	118,670	102,520
Shares granted	87,092	82,450	76,850
Shares vested	(79,678)	(68,500)	(60,700)
Unvested shares at the end of the year	140,034	132,620	118,670
Weighted Average Grant Date Fair Value
Unvested shares at the beginning of the year (in dollars per share)	$ 22.86	$ 21.83	$ 20.04
Shares granted (in dollars per share)	$ 22.38	$ 23.84	$ 24.04
Shares vested (in dollars per share)	$ 22.18	$ 23.36	$ 23.85
Unvested shares at the end of the year (in dollars per share)	$ 23.03	$ 22.86	$ 21.83
Scheduled to vest unvested shares
2013	53,548
2014	41,838
2015	29,249
2016	15,399
Estimated future compensation for the unvested shares	3,310
Closing share price (in dollars per share)	$ 23.64
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	2,000	2,000	2,000
Aggregate market value of shares awarded	205	234	222
Market value of shares awarded to each Trustee	41	47	44
Share Award Plans \| RMR \| Officers and Employees
Share award plans
Common shares awarded	78,492	72,450	66,850
Aggregate market value of shares awarded	$ 1,775	$ 1,732	$ 1,625
Award vesting period	5 years

The aggregate market value of shares newly issued during the reporting period under the plan.

No definition available.

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Represents the closing share price of unvested shares.

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The number of unvested shares, that will vest in four years.

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No definition available.

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The number of unvested shares, that will vest in next fiscal year.

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No definition available.

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The number of unvested shares, that will vest in three years.

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No definition available.

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The number of unvested shares, that will vest in two years.

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No definition available.

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- Definition

Market value of shares, newly issued during the reporting period under the plan, to each individual.

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No definition available.

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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

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-Paragraph 2

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As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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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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- Definition

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 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.

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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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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.

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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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- Definition

Number of share instruments newly issued under a share-based compensation plan.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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