Indebtedness (Narrative) (Details) (USD $)	12 Months Ended			1 Months Ended			3 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Aug. 31, 2011 Fixed Rate Unsecured Debt [Member]	Mar. 31, 2011 Fixed Rate Unsecured Debt [Member]	Jan. 31, 2010 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2010 Fixed Rate Unsecured Debt [Member]	Apr. 01, 2010 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Three Point Seven Five Percent Exchangeable Senior Notes [Member]	Dec. 31, 2011 Variable Rate Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Line of Credit - DRLP [Member]	Dec. 31, 2011 Unsecured Line of Credit - Consolidated Subsidiary [Member]
Interest Paid, Net	$ 261,200,000	$ 246,500,000	$ 224,000,000
Interest rate stated on the debt instrument										6.75%	3.75%
Unsecured Debt	2,616,063,000	2,948,405,000					2,616,063,000	2,616,063,000	2,948,405,000			21,000,000
Early Repayment of Senior Debt									292,200,000
Repayment of corporate unsecured debt	334,432,000	392,597,000	707,016,000	122,500,000	42,500,000	99,800,000	167,600,000
Estimated trading value of fixed rate unsecured debt, minimum percentage								102.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage								120.00%
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														1.14%
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														1.55%
Gains (Losses) on Extinguishment of Debt	0	(16,349,000)	20,700,000						16,300,000
Interest Paid, Capitalized	$ 4,300,000	$ 11,500,000	$ 26,900,000