Fair Value of Instruments (Estimated Fair Value and Carrying Amounts) (Details) (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Revolving credit facility	$ 341,417		$ 333,534
Unsecured senior notes	1,465,587		1,066,030
Exchangeable senior debentures	314,588		353,702
Mortgage Loans	935,485		1,043,188
Other secured loan	10,500		10,500
Long-term Debt	3,067,577		2,806,954
Unamortized discount	9,526
Senior Notes [Member]
Unamortized discount	9,413		8,970
4.125% Exchangeable Senior Debentures due 2026 [Member]
Unamortized discount	113		1,456
Estimated Fair Value [Member]
Revolving credit facility	341,417	[1]	333,534	[1]
Unsecured senior notes	1,535,735	[2],[3]	1,103,983	[2],[3]
Exchangeable senior debentures	500,134	[2],[3]	504,241	[2],[3]
Mortgage Loans	986,894	[3]	1,078,220	[3]
Other secured loan	10,725		10,720
Long-term Debt	3,374,905		3,030,698
Carrying Value [Member]
Revolving credit facility	341,417	[1]	333,534	[1]
Unsecured senior notes	1,465,587	[2],[3]	1,066,030	[2],[3]
Exchangeable senior debentures	314,588	[2],[3]	353,702	[2],[3]
Mortgage Loans	935,485	[3]	1,043,188	[3]
Other secured loan	10,500		10,500
Long-term Debt	$ 3,067,577		$ 2,806,954

[1] The carrying value of our revolving credit facility approximates estimated fair value, due to the short-term nature of this instrument along with the variability of interest rates.

[2] The carrying value of the 2015 Notes, 2020 Notes and 2021 Notes are net of discount of $9,413 and $8,970 in the aggregate as of June 30, 2011 and December 31, 2010, respectively. The carrying values of our exchangeable senior debentures are net of discount of $113 and $1,456 as of June 30, 2011 and December 31, 2010, respectively, related to our 2026 Debentures.

[3] Valuations for our unsecured senior notes and mortgage loans are determined based on the expected future payments discounted at risk-adjusted rates. The 2015 Notes, 2020 Notes and 2021 Notes and exchangeable senior debentures are valued based on quoted market prices.