Debt of the Operating Partnership (Unsecured Senior Notes) (Narrative) (Details) (USD $)	6 Months Ended							6 Months Ended		6 Months Ended		6 Months Ended				6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Prudential Shelf Facility [Member]	Dec. 31, 2010 Prudential Shelf Facility [Member]	Dec. 08, 2010 Prudential Shelf Facility [Member]	Jun. 30, 2011 Prudential Shelf Facility [Member] Series D And E [Member]	Jun. 30, 2011 Prudential Shelf Facility [Member] Series D [Member]	Dec. 31, 2010 Prudential Shelf Facility [Member] Series D [Member]	Jun. 30, 2011 Prudential Shelf Facility [Member] Series E [Member]	Dec. 31, 2010 Prudential Shelf Facility [Member] Series E [Member]	Jun. 30, 2011 Prudential Shelf Facility [Member] Series F [Member]	Dec. 31, 2010 Prudential Shelf Facility [Member] Series F [Member]	Jun. 30, 2011 Senior Notes [Member]	Dec. 31, 2010 Senior Notes [Member]	Jun. 30, 2011 Senior Notes [Member] 5.875% Notes due 2020 [Member]	Dec. 31, 2010 Senior Notes [Member] 5.875% Notes due 2020 [Member]	Jun. 30, 2011 Senior Notes [Member] 4.50% Notes due 2015 [Member]	Dec. 31, 2010 Senior Notes [Member] 4.50% Notes due 2015 [Member]	Jun. 30, 2011 Senior Notes [Member] 5.250% Notes due 2021 [Member]
Notes sale date							January 20, 2010					February 3, 2010
Principal amount of the liability component	$ 3,076,081,000			$ 200,000,000	$ 200,000,000		$ 100,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 17,000,000	$ 17,000,000	$ 1,275,000,000
Interest Rate at June 30, 2011								4.57%		5.73%		4.50%				5.875%		4.50%		5.25%
Notes issuance date																January 28, 2010		July 8, 2010		March 8, 2011
Senior notes														1,265,587,000	866,030,000	500,000,000	500,000,000	375,000,000	375,000,000	400,000,000
Maturity Date								Jan. 20, 2015		Jan. 20, 2017		Feb. 03, 2015				Feb. 01, 2020		Jul. 15, 2015		Mar. 15, 2021
Interest payable dates																February 1 and August 1		January 15 and July 15		March 15 and September 15
Percentage of purchase price paid																98.296%		99.697%		99.775%
Unamortized discount	9,526,000													9,413,000	8,970,000
Unsecured senior notes, net of discount	1,465,587,000		1,066,030,000											1,465,587,000	1,066,030,000
Original issuance discount	1,498,000	2,256,000														8,500,000		1,100,000		900,000
Underwriting commissions and expenses																4,400,000		3,100,000		3,600,000
Net proceeds from senior notes																487,100,000		370,800,000		395,500,000
Prudential Shelf Facility, maximum borrowing capacity					$ 250,000,000	$ 200,000,000
Debt instrument, covenant description																(1) a leverage ratio not to exceed 60%, (2) a secured debt leverage ratio not to exceed 40% and (3) an interest coverage ratio of greater than 1.50, and also requires us to maintain total unencumbered assets of not less than 150% of the aggregate principal amount of unsecured debt		(1) a leverage ratio not to exceed 60%, (2) a secured debt leverage ratio not to exceed 40% and (3) an interest coverage ratio of greater than 1.50, and also requires us to maintain total unencumbered assets of not less than 150% of the aggregate principal amount of unsecured debt. At June 30, 2011		(1) a leverage ratio not to exceed 60%, (2) a secured debt leverage ratio not to exceed 40% and (3) an interest coverage ratio of greater than 1.50, and also requires us to maintain total unencumbered assets of not less than 150% of the aggregate principal amount of unsecured debt
Debt instrument, covenant compliance																we were in compliance with each of these financial covenants.		we were in compliance with each of these financial covenants		At June 30, 2011, we were in compliance with each of these financial covenants