Indebtedness (Narrative) (Details) (USD $)			3 Months Ended			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Fixed Rate Secured Debt [Member] years loans	Mar. 31, 2012 Fixed Rate Unsecured Debt [Member]	Mar. 31, 2012 Variable Rate Unsecured Debt [Member]	Mar. 31, 2012 Unsecured Line of Credit - DRLP [Member]	Mar. 31, 2012 Unsecured Line of Credit - Consolidated Subsidiary [Member]	Mar. 31, 2012 Other 2012 Acquisitions [Member] [Domain]
Estimated rate of fixed rate secured debt, minimum			3.50%
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%
Unsecured notes	2,615,612,000	2,616,063,000			21,000,000
Estimated trading value of fixed rate unsecured debt, minimum percentage				102.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage				124.00%
Debt instrument covenant compliance				We were in compliance with all such covenants as of March 31, 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
Spread over LIBOR						1.25%	0.85%
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							1.74%