Long-Term Debt (Details) (USD $)			3 Months Ended																	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011 7.76% notes, due 2012 thru 2016	Jun. 30, 2012 7.76% notes, due 2012 thru 2016	Dec. 31, 2011 7.76% notes, due 2012 thru 2016	Jun. 30, 2012 5.15% notes, due November 15, 2012	Dec. 31, 2011 5.15% notes, due November 15, 2012	Jun. 30, 2012 5.00% notes, due October 1, 2015	Dec. 31, 2011 5.00% notes, due October 1, 2015	Jun. 30, 2012 5.15% notes, due March 1, 2018	Dec. 31, 2011 5.15% notes, due March 1, 2018	Jun. 30, 2012 6.50% notes, due April 1, 2018	Dec. 31, 2011 6.50% notes, due April 1, 2018	Jun. 30, 2012 8.13% notes, due June 1, 2019	Dec. 31, 2011 8.13% notes, due June 1, 2019	Jun. 30, 2012 4.88% notes, due November 15, 2021	Dec. 31, 2011 4.88% notes, due November 15, 2021	Jun. 30, 2012 7.75% debentures, due July 15, 2026	Dec. 31, 2011 7.75% debentures, due July 15, 2026	Jun. 30, 2012 Medium-term notes, 8.7% to 9.0% Series A, due 2014 thru 2021	Dec. 31, 2011 Medium-term notes, 8.7% to 9.0% Series A, due 2014 thru 2021	Jun. 30, 2012 Medium-term notes, 7.3% to 7.6% Series B, due 2013 thru 2023	Dec. 31, 2011 Medium-term notes, 7.3% to 7.6% Series B, due 2013 thru 2023	Jun. 30, 2012 Medium-term notes, 7.6% Series C, due 2018	Dec. 31, 2011 Medium-term notes, 7.6% Series C, due 2018
Long term debt
Long-term debt	$ 2,736,683,000	$ 2,746,942,000		$ 43,483,000	$ 53,742,000	$ 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	$ 750,000,000	$ 750,000,000	$ 115,000,000	$ 115,000,000	$ 40,200,000	$ 40,200,000	$ 30,000,000	$ 30,000,000	$ 8,000,000	$ 8,000,000
Less debt payable within one year	229,944,000	219,315,000
Total long-term debt	2,506,739,000	2,527,627,000
Debt instrument, stated interest rate percentage				7.76%	7.76%	5.15%	5.15%	5.00%	5.00%	5.15%	5.15%	6.50%	6.50%	8.13%	8.13%	4.88%	4.88%	7.75%	7.75%					7.60%	7.60%
Debt instrument, stated interest rate percentage minimum																				8.70%	8.70%	7.30%	7.30%
Debt instrument, stated interest rate percentage maximum																				9.00%	9.00%	7.60%	7.60%
Aggregate principal amount			57,100,000
Aggregate maturities in 2012	209,800,000
Aggregate maturities in 2013	23,200,000
Aggregate maturities in 2014	11,200,000
Aggregate maturities in 2015	166,000,000
Aggregate maturities in 2016	$ 3,000,000