Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended				12 Months Ended				1 Months Ended		12 Months Ended				1 Months Ended		12 Months Ended			3 Months Ended			12 Months Ended				12 Months Ended				0 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended				12 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended		12 Months Ended		1 Months Ended				1 Months Ended			1 Months Ended		1 Months Ended	3 Months Ended		1 Months Ended	12 Months Ended
	Jan. 31, 2013	Jul. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Dec. 31, 2012 sqft	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Maximum	Aug. 31, 2012 Mortgages Property item	Dec. 31, 2012 Mortgages Property	Dec. 31, 2012 IL Pooling Agreement item	Dec. 31, 2012 MOBs	Dec. 31, 2011 MOBs	Aug. 31, 2012 Acquisition Senior living communities New York item	Dec. 31, 2012 Acquisition Senior living communities Tennessee item	Dec. 31, 2012 Acquisition Senior living communities Texas item	Dec. 31, 2012 Acquisition MOBs item sqft	Dec. 31, 2011 Acquisition MOBs item sqft	Dec. 31, 2012 Operations Transfer Agreement TRS item	Mar. 31, 2013 RMR	Mar. 31, 2012 RMR	Mar. 31, 2011 RMR	Dec. 31, 2012 RMR item	Dec. 31, 2011 RMR	Dec. 31, 2010 RMR	Dec. 31, 2012 RMR MOBs	Dec. 31, 2012 RMR Restricted shares	Dec. 31, 2011 RMR Restricted shares	Dec. 31, 2010 RMR Restricted shares	Dec. 31, 2012 RMR Regional Offices item sqft	Aug. 31, 2012 Five Star item	Apr. 30, 2010 Five Star	Jun. 30, 2011 Five Star	Dec. 31, 2012 Five Star item	Dec. 31, 2011 Five Star	Dec. 31, 2010 Five Star	May 31, 2011 Five Star	Dec. 31, 2012 Five Star AL Pooling Agreement item	May 31, 2012 Five Star Senior living communities Property	Dec. 31, 2012 Five Star Senior living communities item	Aug. 31, 2012 Five Star Senior living communities	Feb. 29, 2012 Five Star Senior living communities Alabama	Dec. 31, 2012 Five Star Rehabilitation hospitals item	Dec. 31, 2012 Five Star Acquisition	Dec. 31, 2011 Five Star Acquisition	Jul. 31, 2012 Five Star Acquisition Senior living communities	May 31, 2012 Five Star Acquisition Senior living communities	Jul. 31, 2012 Five Star Acquisition Senior living communities South Carolina item	May 31, 2012 Five Star Acquisition Senior living communities South Carolina item	Aug. 31, 2012 Five Star Acquisition Senior living communities Missouri item	Aug. 31, 2012 Five Star Acquisition Senior living communities New York item	Feb. 29, 2012 Five Star Acquisition Senior living communities Alabama item	May 31, 2012 Five Star Operations Transfer Agreement TRS item	Dec. 31, 2012 CommonWealth REIT (CWH) item	Sep. 30, 2011 CommonWealth REIT (CWH) Acquisition MOBs item sqft	Mar. 31, 2011 CommonWealth REIT (CWH) Acquisition MOBs item	Jan. 31, 2011 CommonWealth REIT (CWH) Acquisition MOBs sqft	Jun. 30, 2012 AIC	Dec. 31, 2012 AIC	Dec. 31, 2011 AIC	Dec. 31, 2010 AIC	Dec. 31, 2012 Sunrise Senior Living, Inc. Operations Transfer Agreement item
Related person transactions
Number of agreements to avail management and administrative services																							2
Number of employees																							820
Number of Managing Trustees also serving as managing trustee of CWH																																		1																				2
Annual rate of compensation payable under business management agreement as a percentage of book value of the assets owned by the entity or its subsidiaries as of October 12, 1999																							0.50%
Annual rate of compensation payable under business management agreement as a percentage of historical cost of other real estate investments up to the first $250,000 (as a percent)																							0.70%
Historical cost of other real estate investments used to determine the compensation payable to related parties																							$ 250,000
Annual rate of compensation payable under business management agreement as a percentage of historical cost of other real estate investments over $250,000 (as a percent)																							0.50%
Incentive fee (as a percent)																							15.00%
Incentive fee (in dollars per share)								$ 0.02
Business management agreement expense																							25,372	20,814	17,166
Common shares issued to RMR	11,500,000	13,800,000	9,200,000	11,500,000																21,968	20,462	8,939
Management fees payable under property management agreement as a percentage of gross rents (as a percent)																										3.00%															5.00%
Construction supervision fees payable under property management agreement as a percentage of construction costs																										5.00%
Property management agreement expense																							5,791	4,455	2,353									5,582	835
Pro rata share of internal audit costs																							193	240	211
Period by which business management agreement and property management agreement get automatically renewed																							1 year
Period before which the written notice is required to be given for cancellation of business management agreement and the property management agreement																							60 days
Number of business days before which the notice is required to be given for termination of property management agreement																							5 days
Area of real estate properties leased (in square feet)					636,000																									4,100
Number of properties agreed to be acquired, or agreed to be acquired, referred as leased properties																														1
Rental revenue from related party																							186	180	169
Awards granted (in shares)																											78,492	72,450	66,850
Aggregate value of awards granted during the period																											1,775	1,732	1,625
Portion of the awards granted that vested on grant date																											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
Beneficial ownership of common shares (as a percent)																																		100.00%
Investment in common shares																																		4,235,000																				250,000
Investment in common shares purchased from underwriters																																	1,000,000
Value of investment in common shares purchased from underwriters																																	5,000
Percentage of total shares outstanding																																		8.80%
Minimum percentage of ownership of lessee's voting stock above which the entity has the option to cancel all its rights, minimum																																		9.80%
Number of real estate properties leased																			10																					188			2
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
Minimum annual rent payable to entity																																		197,688
Total rental income recognized																																		200,912	195,409	189,682
Rents receivable																																		17,680	17,313
Real estate improvements purchased																																		30,520	33,269	31,894								30,520	33,269
Increase or decrease in annual lease rent payable																																		2,456	2,665	2,555								2,456	2,665
Management fees as a percentage of gross revenues																																		3.00%
Annual return as a percentage of invested surplus specified as a base for determining incentive fee																																		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%
Renewal period																															5 years			15 years
Number of consecutive renewal terms of agreement																															9			2
Maximum percentage of any class of equity shares that can be acquired without approval																																		9.80%
Number of pooling agreement																																		3				2
Number of communities that include assisted living units																																						20
Number of properties acquired, referred to as managed properties																																		39						39
Number of living units														310	90	78																																232	59	87	310	92
Aggregate purchase price of properties acquired, excluding closing costs												449,657	991,618		11,550	9,000	225,695	336,395																								11,300				37,273	8,059			11,280	99,000				167,000	470,000
Number of properties under accelerated lease termination																																																														10
Number of communities under management operations																																																					10
Amount agreed to be lent under Bridge Loan Agreement																																					80,000
Interest and other income					1,117	1,451	844																											314	593
Early repayment of debt variable portion									199,197
Number of Properties released from mortgage as collateral									11
Number of properties mortgaged									28	56
Number of Independent Trustees also serving as independent trustee of CWH																																																						1
Proceeds from repayment of bridge loan																																38,000
Minimum percentage of common stock required by transaction agreement																																																						10.00%
Area of real estate properties acquired (in square feet)																	839,194	2,480,159																																					1,310,000		2,803,000
Ownership percentage held by us, RMR, Five Star, CWH and four other companies																																																											12.50%
The entity owns less than this percentage of an equity method investment																																																											20.00%
Number of properties acquired or agreed to be acquired																	13	28																					6
Number of properties agreed to be acquired, referred as leased properties																																																							13	27
Amount invested in equity investee																																																											5,209
Equity method investments, carrying value																																																											5,629	5,291
Earnings and losses recognized related to equity investments					316	139	(1)																																																				316	139	(1)
Period for which property insurance program was extended																																																										1 year
Coverage of property insurance																																																											500,000
Total premiums including taxes and fees																																																										$ 4,438		$ 1,600	$ 275
Number of communities that include independent living units											2