Long-Term Debt (Details) (USD $)	6 Months Ended	12 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Long term debt
Long-term debt	$ 2,505,286,000	$ 2,526,173,000
Less debt payable within one year	6,212,000	23,204,000
Total long-term debt	2,499,074,000	2,502,969,000
Aggregate maturities for the second half of 2013	3,000,000
Aggregate maturities in 2014	11,200,000
Aggregate maturities in 2015	166,000,000
Aggregate maturities in 2016	3,000,000
Aggregate maturities in 2017	0
7.76% notes, due 2013 thru 2016
Long term debt
Long-term debt	22,086,000	32,973,000
Debt instrument, stated interest rate percentage	7.76%	7.76%
5.00% notes, due October 1, 2015
Long term debt
Long-term debt	150,000,000	150,000,000
Debt instrument, stated interest rate percentage	5.00%	5.00%
5.15% notes, due March 1, 2018
Long term debt
Long-term debt	200,000,000	200,000,000
Debt instrument, stated interest rate percentage	5.15%	5.15%
6.50% notes, due April 1, 2018
Long term debt
Long-term debt	500,000,000	500,000,000
Debt instrument, stated interest rate percentage	6.50%	6.50%
8.13% notes, due June 1, 2019
Long term debt
Long-term debt	700,000,000	700,000,000
Debt instrument, stated interest rate percentage	8.13%	8.13%
4.88% notes, due November 15, 2021
Long term debt
Long-term debt	750,000,000	750,000,000
Debt instrument, stated interest rate percentage	4.88%	4.88%
7.75% debentures, due July 15, 2026
Long term debt
Long-term debt	115,000,000	115,000,000
Debt instrument, stated interest rate percentage	7.75%	7.75%
Medium-term notes, 7.3% to 7.6% Series B, due 2013 thru 2023
Long term debt
Long-term debt	20,000,000	30,000,000
Debt instrument, stated interest rate percentage minimum	7.30%	7.30%
Debt instrument, stated interest rate percentage maximum	7.60%	7.60%
Medium-term notes, 8.7% to 9.0% Series A, due 2014 thru 2021
Long term debt
Long-term debt	40,200,000	40,200,000
Debt instrument, stated interest rate percentage minimum	8.70%	8.70%
Debt instrument, stated interest rate percentage maximum	9.00%	9.00%
Medium-term notes, 7.6% Series C, due 2018
Long term debt
Long-term debt	$ 8,000,000	$ 8,000,000
Debt instrument, stated interest rate percentage	7.60%	7.60%

Interest rate stated in the contractual debt agreement.

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