Long-Term Debt (Details) (USD $)	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Long term debt
Long-term debt	$ 2,501,879,000	$ 2,526,173,000
Less debt payable within one year	6,162,000	23,204,000
Total long-term debt	2,495,717,000	2,502,969,000
Aggregate maturities for the remainder of 2013	0
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	18,679,000	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%

Contractual interest rate for funds borrowed, under the debt agreement.

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