Dispositions, Assets Held for Sale and Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real property dispositions:
Property dispositions	$ 186,050	$ 184,382	$ 186,713
Add: Gain on sales of real property	61,160	36,115	43,394
Proceeds from sales of real property	247,210	219,027	224,007
Revenues:
Rental income	16,134	45,219	65,296
Other income	0	0	8,059
Expenses:
Interest expense	3,604	9,664	12,916
Property operating expenses	4,394	7,172	6,464
Provision for depreciation	5,199	15,580	24,231
Income (loss) from discontinued operations, net	2,937	12,803	29,744
Senior housing facilities [Member]
Real property dispositions:
Property dispositions	150,755	170,290	101,155
Medical office buildings [Member]
Real property dispositions:
Property dispositions	35,295	14,092	85,558
Land parcels [Member]
Real property dispositions:
Property dispositions	$ 0	$ 0	$ 0