Notes Payable (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Debt instrument [Abstract]
Notes payable	$ 1,750,000,000	$ 1,750,000,000
Notes Payable Due March 2013 [Member]
Debt instrument [Abstract]
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
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
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
Issuance date	September 2005
Maturity date	September 2017
Interest rate (in hundredths)	5.375%
Notes Payable Due August 2019 [Member]
Debt instrument [Abstract]
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
Issuance date	June 2010
Maturity date	January 2021
Interest rate (in hundredths)	5.75%
Bonds Payable Due March 2035 [Member]
Debt instrument [Abstract]
Maturity date	March 2035
Interest rate (in hundredths)	5.875%
Bonds Payable Due March 2035 second issuance June 2011 [Member]
Debt instrument [Abstract]
Issued notes payable	150,000,000	150,000,000
Issuance date	June 2011
Bonds Payable Due March 2035 first issuance March 2005 [Member]
Debt instrument [Abstract]
Issued notes payable	$ 100,000,000	$ 100,000,000
Issuance date	March 2005