Indebtedness (Narrative) (Details) (USD $)	6 Months Ended			6 Months Ended		3 Months Ended	6 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Fixed Rate Secured Debt [Member] years loans	Jun. 30, 2012 Variable Rate Secured Debt [Member]	Jun. 30, 2012 Fixed Rate Unsecured Debt [Member]	Jun. 30, 2012 Fixed Rate Unsecured Debt [Member]	Jun. 30, 2012 Variable Rate Unsecured Debt [Member]	Jun. 30, 2012 Unsecured Line of Credit - DRLP [Member]	Jun. 30, 2012 Unsecured Line of Credit - Consolidated Subsidiary [Member]	Jun. 30, 2012 Other 2012 Acquisitions [Member] [Domain]
Estimated rate of fixed rate secured debt, minimum				3.30%
Estimated rate of fixed rate secured debt, maximum				5.40%
Secured loans acquired				1
Face value from acquisition				$ 18,100,000
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt											18,741,000
Assumed weighted average interest rate				5.14%
Weighted average remaining term (years)				2.2
Estimated effective interest rate of loan				3.50%		4.466%	4.466%
Secured Debt, Variable Rate					13,300,000
Debt Instrument, Basis Spread on Variable Rate					2.50%				1.25%	0.85%
Debt Instrument, Interest Rate at Period End					2.75%
Number of secured loans repaid				4
Repayments of Secured Debt	102,869,000	7,968,000		95,800,000
Debt, Weighted Average Interest Rate				6.02%
Proceeds from Unsecured Debt Issuance	300,000,000	0				300,000,000
Debt Instrument, Interest Rate, Stated Percentage						4.375%	4.375%
Unsecured notes	2,915,155,000		2,616,063,000					21,000,000
Estimated trading value of fixed rate unsecured debt, minimum percentage							101.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage							126.00%
Debt instrument covenant compliance							We were in compliance with all such covenants as of June 30, 2012		we were in compliance with all covenants under this line of credit.
Borrowing capacity of unsecured lines of credit									850,000,000	30,000,000
Line of credit balance									0	20,293,000
Interest rate on borrowings										1.09%
Maturity date of unsecured line of credit									December 2015	July 2012
Additional borrowings under unsecured lines of credit									400,000,000
Maximum borrowing capacity of unsecured lines of credit									$ 1,250,000,000
Current market rate of unsecured lines of credit										1.75%