Acquisitions of Real Estate Property (Details 2) (USD $)							12 Months Ended
	Dec. 31, 2012	Aug. 31, 2012	Sep. 30, 2011	May 12, 2011	Sep. 30, 2011 MOB Operations	Sep. 30, 2011 Operating Assets	Dec. 31, 2011 NHP	Sep. 30, 2011 NHP
Business acquisition
Conversion ratio of acquired entity common stock to parent common stock (in shares)							0.7866
Amount paid for closing of revolving credit facility							$ 105,000,000
Number of properties acquired							643
Fair values of the assets acquired and liabilities assumed at the date of acquisition
Land and improvements	194,812,000							701,154,000
Buildings and improvements	1,721,060,000							6,147,737,000
Acquired lease intangibles	202,799,000							493,125,000
Investment in unconsolidated entities								93,553,000
Other assets	45,876,000							815,968,000
Total assets acquired	2,190,144,000							8,251,537,000
Notes payable and other debt	412,825,000							1,882,752,000
Other liabilities	127,682,000							720,420,000
Total liabilities assumed	540,507,000							2,603,172,000
Redeemable OP unitholder interests assumed								100,888,000
Noncontrolling interest assumed (including redeemable interests)	37,698,000							76,658,000
Net assets acquired	1,611,939,000							5,470,819,000
Cash acquired	24,871,000							29,205,000
Equity issued								5,365,819,000
Total cash used	1,587,068,000	350,000,000						75,795,000
Goodwill included in other assets			$ 399,000,000	$ 80,500,000	$ 338,500,000	$ 60,500,000