Accounting Policies (Details) (USD $)	1 Months Ended		12 Months Ended
	Jul. 31, 2010 segment	Jun. 30, 2010 segment	Dec. 31, 2012 tranche segment	Dec. 31, 2011	Dec. 31, 2010
Cash Equivalents
Maximum term of maturity to classify investments as cash equivalents (in months)			3 months
Deferred Financing Costs
Deferred financing costs, net of accumulated amortization			$ 42,551,000	$ 26,669,000
Amortized costs included in interest expense			10,500,000	17,800,000	17,800,000
Fair Values of Financial Instruments
Number of foreign currency tranches reported at fair value			2
Number of entity shares issued in exchange for redemption of OP Units			$ 0.7866
Revenue Recognition
Straight-line rent receivables			120,325,000	96,883,000
Term of resident lease agreements, minimum (in months)			12 months
Term of resident lease agreements, maximum (in months)			18 months
Notice period to cancel lease agreements by the resident (in days)			30 days
Segment Reporting
Number of reportable business segments	2	2	3
Business acquisition
Tenant purchase option recorded, net			$ 432,500,000	$ 644,000,000
Building
Business acquisition
Estimated remaining life (in years)			35 years
Lillibridge Acquisition
Business acquisition
Number of Medical office buildings	96