Related Person Transactions (Details) (USD $)	1 Months Ended				12 Months Ended			1 Months Ended	3 Months Ended	1 Months Ended	5 Months Ended	10 Months Ended	6 Months Ended	1 Months Ended							43 Months Ended	1 Months Ended				0 Months Ended		1 Months Ended		0 Months Ended	2 Months Ended
	Feb. 28, 2015	Apr. 30, 2014	Jan. 31, 2013	Jul. 31, 2012	Dec. 31, 2014 sqft item property state	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2012 item	Dec. 31, 2014 property	Jan. 31, 2015 property sqft	Dec. 31, 2013 property	Oct. 31, 2014 property	Sep. 30, 2014 property	Dec. 31, 2014 item	Sep. 30, 2013	Apr. 30, 2012	Jul. 31, 2014 item	Aug. 31, 2013 property	Jun. 30, 2013 property	May 31, 2011 property	Dec. 31, 2014 property item	Dec. 31, 2013 property	Oct. 31, 2014	Jun. 30, 2014 property item	Jan. 31, 2014	May 09, 2014	May 08, 2014	Sep. 30, 2014	Jul. 31, 2013 item	Dec. 23, 2013 item	Feb. 24, 2015
Related person transactions
Number of properties agreed to be offered for sale					5				5					5							5
Number of properties pending acquisition					38				38					38							38
Number of employees					0				0					0							0
Income recognized arising from equity investments					$ 87,000	$ 334,000	$ 316,000
Issuance of shares (in shares)	31,050,000	15,525,000	11,500,000	13,800,000
Area of real estate properties leased (in square feet)					750,000
Interest and other income					425,000	711,000	1,117,000
Number of states in which properties are located					38				38					38							38
Mortgages
Related person transactions
Early repayment of debt variable portion								199,197,000	23,234,000
Number of Properties released from mortgage as collateral								11
Number of Real Estate Properties Collateralized					45			28	45					45							45
Senior living communities
Related person transactions
Number of properties agreed to be offered for sale					4	10			4		10			4							4	10
Number of living units					312	744			312		744			312							312	744
MOBs
Related person transactions
Number of properties agreed to be offered for sale					4				4									4
Acquisition | Senior living communities
Related person transactions
Number of properties acquired or agreed to be acquired					2	6
Acquisition | MOBs
Related person transactions
Number of properties acquired or agreed to be acquired					2	6
Purchase price					1,156,963,000	117,475,000
Area of real estate properties (in square feet)					1,776,000	385,171,000			1,776,000		385,171,000			1,776,000							1,776,000	385,171,000
Acquisition | MOBs | Subsequent event
Related person transactions
Number of properties acquired or agreed to be acquired					23					23
Purchase price										539,000,000
Area of real estate properties (in square feet)										2,200,000
Acquisition | MOBs | Texas
Related person transactions
Number of properties acquired or agreed to be acquired		1
Purchase price		32,932,000
Area of real estate properties (in square feet)		125,000
Sale
Related person transactions
Sale price of properties under agreement to be sold											92,550,000	15,650,000
Number of properties sold or agreed to be sold											3	6
Sale | Senior living communities
Related person transactions
Number of properties sold or agreed to be sold					6
Sale | Rehabilitation hospitals
Related person transactions
Number of properties sold or agreed to be sold						2					2
Sale | MOBs
Related person transactions
Sale price of properties under agreement to be sold													11,675,000
Number of properties sold or agreed to be sold					3								3
Agreement to acquire | Senior living communities
Related person transactions
Number of living units					3,466				3,466					3,466							3,466
Purchase price														790,000,000
Five Star
Related person transactions
Beneficial ownership of common shares (as a percent)					100.00%				100.00%					100.00%							100.00%
Investment in common shares					4,235,000				4,235,000					4,235,000							4,235,000
Percentage of total shares outstanding					8.60%				8.60%					8.60%							8.60%
Minimum annual rent payable to entity					190,663,000
Total rental income recognized					196,269,000	203,724,000	200,912,000
Rents receivable					17,310,000	17,960,000	17,680,000		17,310,000		17,960,000			17,310,000							17,310,000	17,960,000
Real estate improvements purchased					25,804,000	27,208,000	30,520,000
Increase or decrease in annual lease rent payable					2,066,000	2,177,000	2,456,000								(9,500,000)
Number of combination or pooling agreements					4				4					4							4
Property management agreement expense					9,765,000	9,229,000	5,582,000
Minimum percentage of ownership of lessee's voting stock above which the entity has the option to cancel all its rights, minimum					9.80%				9.80%					9.80%							9.80%
Annual return as a percentage of invested surplus specified as a base for determining incentive fee					35.00%				35.00%					35.00%							35.00%
Incentive fee as percentage of the annual net operating income after the entity realizes an annual return equal to 8% of invested capital					8.00%				8.00%					8.00%							8.00%
Renewal period					15 years			5 years
Maximum percentage of any class of equity shares that can be acquired without approval					9.80%				9.80%					9.80%							9.80%
Number of consecutive period during which the entity must not receive the minimum return for the property management agreement to be subject to the pooling agreement					3 years
Number of consecutive renewal terms of agreement					2			9
Management fees as a percentage of gross revenues					3.00%				3.00%					3.00%							3.00%
Amount agreed to be lent under Bridge Loan Agreement																				80,000,000
Interest and other income							314,000
Proceeds from repayment of bridge loan																38,000,000
Incentive fee payable					0				0					0							0
Five Star | AL Pooling Agreement
Related person transactions
Number of combination or pooling agreements					3				3					3							3
Number of communities that include assisted living units					20
Five Star | IL Pooling Agreement
Related person transactions
Number of communities that include assisted living units																	2
Number of communities that include independent living units					2
Five Star | Second AL Pooling Agreement
Related person transactions
Number of communities that include assisted living units					19												19
Five Star | Third AL Pooling Agreement
Related person transactions
Number of communities that include assisted living units					3
Five Star | Senior living communities
Related person transactions
Number of real estate properties leased					181				181					181							181
Increase or decrease in annual lease rent payable																		(255,000)
Number of properties sold or agreed to be sold																		1
Number of properties agreed to be sold																			11
Number of properties acquired, referred to as managed properties					46
Number of properties acquired or agreed to be acquired					46									2						6	36
Purchase price														47,430,000
Management fees as a percentage of gross revenues					5.00%				5.00%					5.00%							5.00%
Five Star | Rehabilitation hospitals
Related person transactions
Number of properties sold or agreed to be sold																						2
Five Star | Acquisition | Senior living communities | New York
Related person transactions
Number of living units					310				310					310							310
Five Star | Sale | Senior living communities
Related person transactions
Increase or decrease in annual lease rent payable																							285,000	(452,000)	210,000
Sale price of properties under agreement to be sold																		2,550,000						4,500,000
Number of properties sold or agreed to be sold																								2
Number of living units																		112						156
Five Star | Sale | Senior living communities | Subsequent event
Related person transactions
Increase or decrease in annual lease rent payable	(23,000)
Sale price of properties under agreement to be sold	250,000
Number of living units	120
Five Star | Sale | Senior living communities | Texas
Related person transactions
Sale price of properties under agreement to be sold																									2,400,000
Number of properties agreed to be sold																									1
Number of living units																									36
Five Star | Sale | Senior living communities | Virginia
Related person transactions
Sale price of properties under agreement to be sold																							2,850,000
Number of living units												70											70
Five Star | Sale | Senior living communities | Arizona
Related person transactions
Increase or decrease in annual lease rent payable																							(590,000)
Sale price of properties under agreement to be sold																							5,900,000
Number of properties sold or agreed to be sold																							2
Number of living units												177											177
Five Star | Purchase Agreement | Rehabilitation hospitals
Related person transactions
Sale price of properties under agreement to be sold																						90,000,000
Number of properties sold or agreed to be sold																						2
D&R Yonkers LLC
Related person transactions
Management fees as a percentage of gross revenues					3.00%				3.00%					3.00%							3.00%
Incentive fee payable					0				0					0							0
Sublease income receivable					3,008,000				3,008,000					3,008,000							3,008,000
Sublease rental					2,949,000	2,863,000	945,000
RMR
Related person transactions
Property management agreement expense					8,259,000	6,568,000	5,791,000
Number of agreements to avail management and administrative services					2				2					2							2
Period before which the written notice is required to be given for cancellation of business management agreement and the property management agreement																										120 days	60 days
Number of business days before which the notice is required to be given for termination of property management agreement																											5 days
Notice period subject to approval by majority vote of independent trustees																											60 days
Termination fee ratio																											2.75
Period on which termination fees computation is based																											24 months
Period Within Which Property Management Agreement To Be Renewed Or Terminated																											12 months
Additional termination fee ratio																											12
Period for providing transition services under business management agreement and property management agreement					120 days
Business management agreement expense					30,794,000	27,013,000	25,372,000
Number of entities to which RMR provides management services																													3
Premium paid for combined and separate directors' and officers' liability insurance policy																								51,000					343,000
Issuance of shares (in shares)					123,462	21,968	20,462
Liability insurance policy term																												2 years
Aggregate coverage of combined directors' and officers' liability insurance policy purchased by the related party																													10,000,000
Additional liability insurance policy term					1 year
Aggregate additional coverage of combined directors' and officers' liability insurance policy purchased by the related party					20,000,000				20,000,000					20,000,000							20,000,000
Non-indemnifiable coverage of combined directors' and officers' liability insurance policy purchased by the related party					5,000,000
Premium payable for combined directors' and officers' liability insurance policy					685,000				685,000					685,000							685,000
Incentive fee earned					0	0
Base management fee payable as a percentage of aggregate book value of real estate assets or transferred assets						0.50%	0.50%				0.50%											0.50%
Base management fee payable as a percentage of average historical cost of real estate investments, excluding transferred assets for investments up to specified amount						0.70%	0.70%				0.70%											0.70%
Base management fee payable as a percentage of historical cost of other real estate investments, excluding transferred assets for investments after specified amount						0.50%	0.50%				0.50%											0.50%
Percentage of incentive fee payable						15.00%	15.00%				15.00%											15.00%
Cap on the value of incentive fee						$ 0.02	$ 0.02				$ 0.02											0.02
Pro rata share of internal audit costs					262,000	201,000	193,000
Period by which business management agreement and property management agreement get automatically renewed					1 year
Rental revenue from related party							186,000
Aggregate coverage of separate directors' and officers' liability insurance policy purchased																													5,000,000
RMR | Maximum
Related person transactions
Threshold amount of real estate investments for payment of base management fee						250,000,000	250,000,000				250,000,000											250,000,000
RMR | Restricted shares
Related person transactions
Awards granted (in shares)					81,700	82,600	78,492
Aggregate value of awards granted during the period					1,750,000	1,888,000	1,775,000
Portion of the awards granted that vested on grant date (as a percent)					20.00%
Portion of the awards granted which will vest on each of the next four anniversaries of the grant date					20.00%
Award vesting period					4 years
RMR | Amended Agreement
Related person transactions
Base business management fee payable in common shares (as a percent)																														10.00%
Base management fee payable as a percentage of aggregate book value of real estate assets or transferred assets					0.50%				0.50%					0.50%							0.50%
Base management fee payable as a percentage of average historical cost of real estate investments, excluding transferred assets for investments up to specified amount					0.70%				0.70%					0.70%							0.70%
Base management fee payable as a percentage of average historical cost of real estate investments, excluding transferred assets for investments exceeding specified amount																														0.50%
Threshold amount of other real estate investments for payment of base management fee																														250,000,000.000
Base management fee payable as a percentage of average closing price per share of common shares on NYSE																														0.70%
Base management fee payable as a percentage of average market capitalization exceeding specified amount																														0.50%
Threshold amount of average market capitalization for payment of base management fee																														250,000,000.000
Base business management fee payable in cash (as a percent)																														90.00%
Incentive management fee payable (as a percent)																														12.00%
Portion of shares issued in payment of an incentive management fee (as a percent)																														33.33%
Portion of remaining shares to be vested in equal annual installments (as a percent)																														66.67%
Number of equal annual installments for vesting of common shares																														2
Number of times registration right can be exercisable																														2
RMR | Amended Agreement | Maximum
Related person transactions
Threshold amount of real estate investments for payment of base management fee																														250,000,000
Base management fee payable, average market capitalization																														250,000,000
RMR | Subsequent event
Related person transactions
Issuance of shares (in shares)																															25,263
RMR | Regional Offices
Related person transactions
Area of real estate properties leased (in square feet)							4,100
Number of properties agreed to be acquired, or agreed to be acquired, referred as leased properties							1
RMR | MOBs
Related person transactions
Construction supervision fees payable under property management agreement as a percentage of construction costs					5.00%				5.00%					5.00%							5.00%
AIC
Related person transactions
Ownership percentage held by the entity and other shareholders																										14.30%
Term of property insurance policy					1 year	1 year	1 year
Total premiums including taxes and fees					3,118,000	4,748,000	4,438,000
The entity owns less than this percentage of an equity method investment					20.00%				20.00%					20.00%							20.00%
Amount invested in equity investee					6,034,000				6,034,000					6,034,000							6,034,000
Equity method investments, carrying value					6,827,000	5,913,000	5,629,000		6,827,000		5,913,000			6,827,000							6,827,000	5,913,000
Income recognized arising from equity investments					87,000	334,000	316,000
Coverage of property insurance					500,000,000	500,000,000	500,000,000
Number of shares purchased																										2,857
Payments for share purchased																										825,000
SIR
Related person transactions
Period for which the Company has agreed not to make Unsolicited Proposals to Acquire					36 months
SIR | Healthcare properties
Related person transactions
Sale price of properties under agreement to be sold					539,000,000
Debt disposed of from properties sold					$ 30,000,000
Number of properties agreed to be acquired					23
Area of real estate properties (in square feet)					2,200,000				2,200,000					2,200,000							2,200,000
Number of states in which properties are located					12				12					12							12