Debt of the Operating Partnership (Unsecured Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Subtotal	$ 3,076,081
Unamortized discounts	(9,526)
Unsecured senior notes, net of discount	1,465,587	1,066,030
Prudential Shelf Facility [Member]
Subtotal	200,000	200,000
Prudential Shelf Facility [Member] | Series A [Member]
Interest Rate at June 30, 2011	7.00%
Maturity Date	Jul. 24, 2011
Subtotal	25,000	25,000
Prudential Shelf Facility [Member] | Series B [Member]
Interest Rate at June 30, 2011	9.32%
Maturity Date	Nov. 05, 2013
Subtotal	33,000	33,000
Prudential Shelf Facility [Member] | Series D [Member]
Interest Rate at June 30, 2011	4.57%
Maturity Date	Jan. 20, 2015
Subtotal	50,000	50,000
Prudential Shelf Facility [Member] | Series C [Member]
Interest Rate at June 30, 2011	9.68%
Maturity Date	Jan. 06, 2016
Subtotal	25,000	25,000
Prudential Shelf Facility [Member] | Series E [Member]
Interest Rate at June 30, 2011	5.73%
Maturity Date	Jan. 20, 2017
Subtotal	50,000	50,000
Prudential Shelf Facility [Member] | Series F [Member]
Interest Rate at June 30, 2011	4.50%
Maturity Date	Feb. 03, 2015
Subtotal	17,000	17,000
Senior Notes [Member]
Subtotal	1,275,000
Unamortized discounts	(9,413)	(8,970)
Principal Outstanding (Senior notes)	1,265,587	866,030
Unsecured senior notes, net of discount	1,465,587	1,066,030
Senior Notes [Member] | 5.875% Notes due 2020 [Member]
Interest Rate at June 30, 2011	5.875%
Maturity Date	Feb. 01, 2020
Principal Outstanding (Senior notes)	500,000	500,000
Senior Notes [Member] | 4.50% Notes due 2015 [Member]
Interest Rate at June 30, 2011	4.50%
Maturity Date	Jul. 15, 2015
Principal Outstanding (Senior notes)	375,000	375,000
Senior Notes [Member] | 5.250% Notes due 2021 [Member]
Interest Rate at June 30, 2011	5.25%
Maturity Date	Mar. 15, 2021
Principal Outstanding (Senior notes)	$ 400,000