Long-Term Debt (Details) (USD $)			3 Months Ended														9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 7.76% notes, due 2012 thru 2016	Sep. 30, 2011 7.76% notes, due 2012 thru 2016	Sep. 30, 2011 5.15% notes, due November 15, 2012	Dec. 31, 2010 5.15% notes, due November 15, 2012	Sep. 30, 2011 5.00% notes, due October 1, 2015	Dec. 31, 2010 5.00% notes, due October 1, 2015	Sep. 30, 2011 5.15% notes, due March 1, 2018	Dec. 31, 2010 5.15% notes, due March 1, 2018	Sep. 30, 2011 6.50% notes, due April 1, 2018	Dec. 31, 2010 6.50% notes, due April 1, 2018	Sep. 30, 2011 8.13% notes, due June 1, 2019	Dec. 31, 2010 8.13% notes, due June 1, 2019	Sep. 30, 2011 7.75% debentures, due July 15, 2026	Dec. 31, 2010 7.75% debentures, due July 15, 2026	Sep. 30, 2011 Medium-term notes, 8.5% to 9.0% Series A, due 2011 thru 2021	Dec. 31, 2010 Medium-term notes, 8.5% to 9.0% Series A, due 2011 thru 2021	Sep. 30, 2011 Medium-term notes, 7.3% to 7.6% Series B, due 2013 thru 2023	Dec. 31, 2010 Medium-term notes, 7.3% to 7.6% Series B, due 2013 thru 2023	Sep. 30, 2011 Medium-term notes, 7.6% Series C, due 2018	Dec. 31, 2010 Medium-term notes, 7.6% Series C, due 2018
Long term debt
Long-term debt	$ 2,003,911,000	$ 1,949,200,000		$ 54,711,000	$ 200,000,000	$ 200,000,000	$ 150,000,000	$ 150,000,000	$ 200,000,000	$ 200,000,000	$ 500,000,000	$ 500,000,000	$ 700,000,000	$ 700,000,000	$ 115,000,000	$ 115,000,000	$ 46,200,000	$ 46,200,000	$ 30,000,000	$ 30,000,000	$ 8,000,000	$ 8,000,000
Less debt payable within one year	25,315,000	6,000,000
Total long-term debt	1,978,596,000	1,943,200,000
Debt instrument, stated interest rate percentage				7.76%	5.15%	5.15%	5.00%	5.00%	5.15%	5.15%	6.50%	6.50%	8.13%	8.13%	7.75%	7.75%					7.60%	7.60%
Debt instrument, stated interest rate percentage minimum (as a percent)																	8.50%	8.50%	7.30%	7.30%
Debt instrument, stated interest rate percentage maximum (as a percent)																	9.00%	9.00%	7.60%	7.60%
Aggregate principal amount			57,100,000
Aggregate maturities in 2011	6,000,000
Aggregate maturities in 2012	219,300,000
Aggregate maturities in 2013	23,200,000
Aggregate maturities in 2014	11,200,000
Aggregate maturities in 2015	$ 166,000,000