Notes Payable (Details) (USD $)	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Debt Instrument [Abstract]
Notes payable	$ 1,750,000,000	$ 1,600,000,000
Notes payable	1,750,000,000	1,600,000,000
Property acquisitions announced total	544,000,000
Notes Payable Due March 2013 [Member]
Debt Instrument [Abstract]
Notes payable	100,000,000	100,000,000
Notes payable	100,000,000	100,000,000
Issuance date	March 2003
Maturity date	March 2013
Interest Rate (in hundredths)	5.375%
Notes Payable Due November 2015 [Member]
Debt Instrument [Abstract]
Notes payable	150,000,000	150,000,000
Notes payable	150,000,000	150,000,000
Issuance date	November 2003
Maturity date	November 2015
Interest Rate (in hundredths)	5.50%
Notes Payable Due September 2016 [Member]
Debt Instrument [Abstract]
Notes payable	275,000,000	275,000,000
Notes payable	275,000,000	275,000,000
Issuance date	September 2006
Maturity date	September 2016
Interest Rate (in hundredths)	5.95%
Notes Payable Due September 2017 [Member]
Debt Instrument [Abstract]
Notes payable	175,000,000	175,000,000
Notes payable	175,000,000	175,000,000
Issuance date	September 2005
Maturity date	September 2017
Interest Rate (in hundredths)	5.375%
Nores Payable Due August 2019 [Member]
Debt Instrument [Abstract]
Notes payable	550,000,000	550,000,000
Notes payable	550,000,000	550,000,000
Issuance date	September 2007
Maturity date	August 2019
Interest Rate (in hundredths)	6.75%
Notes Payable Due January 2021 [Member]
Debt Instrument [Abstract]
Notes payable	250,000,000	250,000,000
Notes payable	250,000,000	250,000,000
Issuance date	June 2010
Maturity date	January 2021
Interest Rate (in hundredths)	5.75%
Percentage Price to the investor (in hundredths)	99.404%
Effective yield (in hundredths)	5.826%
Net proceeds from the issuance of Debt	246,300,000
Bonds [Member]
Debt Instrument [Abstract]
Notes payable	250,000,000	100,000,000
Notes payable	250,000,000	100,000,000
Notes Payable 100M issued Due March 2035 [Member]
Debt Instrument [Abstract]
Notes payable	100,000,000
Notes payable	100,000,000
Issuance date	March 2005
Maturity date	March 2035
Interest Rate (in hundredths)	5.875%
Notes Payable 150M issued Due June 2035 [Member]
Debt Instrument [Abstract]
Notes payable	150,000,000
Notes payable	150,000,000
Issuance date	June 2011
Maturity date	March 2035
Interest Rate (in hundredths)	5.875%
Percentage Price to the investor (in hundredths)	94.578%
Effective yield (in hundredths)	6.318%
Net proceeds from the issuance of Debt	$ 140,100,000