Pro Forma Information (unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended				3 Months Ended								12 Months Ended			1 Months Ended						1 Months Ended						1 Months Ended	12 Months Ended			1 Months Ended									1 Months Ended			12 Months Ended			1 Months Ended	12 Months Ended				1 Months Ended	12 Months Ended
	Jan. 31, 2013	Jul. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 item	Dec. 31, 2011 item	Dec. 31, 2010 item	Jan. 31, 2011 Senior unsecured notes 4.30%	Dec. 31, 2012 Senior unsecured notes 4.30%	Dec. 31, 2011 Senior unsecured notes 4.30%	Dec. 31, 2012 Unsecured revolving credit facility	Dec. 31, 2011 Unsecured revolving credit facility	Jun. 30, 2011 Unsecured revolving credit facility	Jun. 30, 2011 Previous unsecured revolving credit facility	Dec. 31, 2010 Previous unsecured revolving credit facility	Dec. 31, 2012 Senior unsecured notes 6.75%	Jul. 31, 2012 Senior unsecured notes 6.75%	Dec. 31, 2011 Senior unsecured notes 6.75%	Apr. 30, 2010 Senior unsecured notes 6.75%	Dec. 31, 2011 Unsecured senior notes due 2021	Dec. 31, 2011 Unsecured senior notes due 2021	Dec. 31, 2012 Unsecured senior notes due 2021	Dec. 31, 2011 Senior unsecured notes 7.875%	Jul. 31, 2012 Senior unsecured notes 5.625% due in 2042	Oct. 31, 2012 Mortgages	Aug. 31, 2012 Mortgages	Jun. 30, 2012 Mortgages	Apr. 30, 2012 Mortgages item	Feb. 29, 2012 Mortgages	Dec. 31, 2012 Mortgages	Dec. 31, 2011 Mortgages	Apr. 30, 2010 Mortgages	Jan. 31, 2012 Senior unsecured notes 8.625%	Dec. 31, 2011 Senior unsecured notes 8.625%	Dec. 31, 2012 Mortgages 6.65%	Dec. 31, 2012 Five Star	Dec. 31, 2011 Five Star	Dec. 31, 2012 Senior living communities Sunrise Senior Living, Inc.	May 31, 2011 Senior living communities Five Star Property	Dec. 31, 2012 MOBs	Dec. 31, 2011 MOBs	Dec. 31, 2012 Acquisition	Dec. 31, 2011 Acquisition	Jul. 31, 2011 Acquisition Senior living communities	Dec. 31, 2012 Acquisition Senior living communities item	Dec. 31, 2011 Acquisition Senior living communities item	Dec. 31, 2012 Acquisition MOBs item	Dec. 31, 2011 Acquisition MOBs item	Dec. 31, 2012 Sale	Dec. 31, 2011 Sale Property	Dec. 31, 2012 Sale MOBs Property	Dec. 31, 2011 Sale MOBs Property	Dec. 31, 2011 Sale Skilled nursing facilities Property	Dec. 31, 2011 Sale Assisted living community Property
Pro Forma Information
Number of properties acquired or agreed to be acquired																																															6						11	28	13	28
Aggregate purchase price of properties acquired, excluding closing costs																																																$ 449,657	$ 991,618			$ 99,808			$ 225,695	$ 336,395
Number of properties sold																																																										7	1	2	4	1
Aggregate sale price																																																									1,100	39,460
(Loss) gain on sale of properties													(101)	21,315	109																																										(101)	21,315
Real estate improvements purchased																																												30,520	33,269
Stated interest rate (as a percent)																4.30%	4.30%	4.30%					7.875%	6.75%		6.75%		6.75%	6.75%	6.75%		5.625%	6.50%	6.40%	6.07%		6.03%				8.625%	8.625%
Debt sold																250,000												300,000	300,000			350,000
Deferred financing fees					12,608				13,760				12,608	13,760	4,661	1,973					6,723		826	44	11,439		2,907	2,487	2,487		1,973	11,439						1,125	2,540	25										12,608	2,540
Unsecured revolving credit facility, maximum borrowing capacity																			750,000		750,000	550,000	550,000
Mortgage debt assumed													121,793	217,317	2,458																																								73,103	13,286
Weighted average interest rate on mortgage debt assumed (as a percent)					5.84%				5.94%				5.84%	5.94%					1.80%	1.70%																6.95%		5.84%					6.65%
Early repayment of debt variable portion																																		199,197
Gain (loss) on extinguishment of debt													6,349	427	2,433							427												6,349
Common shares issued in public offering	11,500,000	13,800,000	9,200,000	11,500,000
Net proceeds from issuance of common shares issued in public offering	262,068	287,052	184,735	247,498									287,052	432,233	281,986
Revenues					194,164	158,633	146,972	145,073	136,603	113,700	101,162	98,552	644,842	450,017	340,113																																			26,840	60,395
Property operating expenses																																																		15,957	29,604
Interest expense													117,240	98,262	80,017																																			2,354	2,915
Number of mortgage loans repaid													20																							17
Repayment of mortgage loans													52,254																				4,152		3,140	32,576	12,386
Repayments of borrowings under revolving credit facility													225,000		98,780																										225,000
Gain (loss) on lease terminations													375																																	375
Impairment of asset charges recorded													3,071	1,990	5,965
Number of properties impaired													1	4	7
Pro forma results of operations
Total revenues													689,703	664,047
Net income													$ 140,846	$ 151,338
Per common share data:
Net income (in dollars per share)													$ 0.80	$ 0.86