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Related Person Transactions - Investment in RMR Inc (Details) $ in Thousands			1 Months Ended		9 Months Ended	12 Months Ended
	Sep. 14, 2017 shares	May 18, 2017 shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 shares	Sep. 30, 2017 USD ($) building shares	Jun. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)
Related person transactions
Common shares granted (in shares)	88,100	3,000	88,100	79,650
RMR Inc | Class A common shares
Related person transactions
Number of shares purchased (in shares)					2,637,408
RMR LLC
Related person transactions
Shares purchased to satisfy tax withholding and payment obligations (in shares)			16,654	17,667
AIC
Related person transactions
Property insurance term					1 year
Number of buildings covered by property insurance | building					2
Equity method investments | $			$ 7,945		$ 7,945		$ 7,116
Scenario, Forecast | AIC
Related person transactions
Equity method investments, property insurance annual premium amount | $						$ 2,433

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- 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.

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- Definition

Number of Buildings Covered by Property Insurance

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- Definition

Represents the term of property insurance policy.

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No definition available.

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- Definition

Represents the number of shares purchased, from related party.

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No definition available.

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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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

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).

+ 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=96867065&loc=d3e5070-113901

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- Definition

The number of shares purchased on the open market during the period for issuance to employees under the plan.

+ 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 (a)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901

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-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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