Other Real Estate Property Investments (Details) (USD $)	9 Months Ended
	Sep. 30, 2011 Facility squarefoot	Sep. 30, 2010 Facility squarefoot
Acquisition
Consideration, Cash Paid	$ 113,830,000	$ 158,795,000
Consideration, Debt Assumed	57,869,000	5,352,000
Consideration, Noncontrolling Interest	1,500,000
Consideration, DownREIT Units		9,267,000
Assets Acquired, Real Estate	159,231,000	164,297,000
Assets Acquired, Net Intangibles	13,968,000	9,117,000
Senior housing
Acquisition
Consideration, Cash Paid		124,935,000
Assets Acquired, Real Estate		124,565,000
Assets Acquired, Net Intangibles		370,000
Life science
Acquisition
Consideration, Cash Paid	84,087,000	21,843,000
Consideration, Debt Assumed	57,869,000
Consideration, DownREIT Units		7,341,000
Assets Acquired, Real Estate	133,040,000	23,998,000
Assets Acquired, Net Intangibles	8,916,000	5,186,000
Payments to acquire productive assets		97,000,000
Number of facilities placed into service	2	4
The square footage of facilities placed in service	88,000	354,000
Medical office
Acquisition
Consideration, Cash Paid	29,743,000	12,017,000
Consideration, Debt Assumed		5,352,000
Consideration, Noncontrolling Interest	1,500,000
Consideration, DownREIT Units		1,926,000
Assets Acquired, Real Estate	26,191,000	15,734,000
Assets Acquired, Net Intangibles	5,052,000	3,561,000
Life science and medical office
Acquisition
Payments to acquire productive assets	$ 87,000,000