Debt (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Short-term Borrowings
Bank line of credit			$ 100,000
Total short-term borrowings	0	0	100,000
Long-term Debt
Total long-term debt including current maturities	2,813,539	2,815,439	2,791,073
Less current maturities of long-term debt	285,152	134,762	5,230
Total long-term debt	2,528,387	2,680,677	2,785,843
Estimated fair value of total long-term debt	2,636,409	2,796,504	2,857,684
6.30% notes due 2013 [Member]
Long-term Debt
Total long-term debt including current maturities	140,382	140,352	140,322
Bank line of credit [Member]
Long-term Debt
Total long-term debt including current maturities	0	0
5.60% notes due 2012 [Member]
Long-term Debt
Total long-term debt including current maturities	134,535	134,508	134,483
10.125% notes due 2015 [Member]
Long-term Debt
Total long-term debt including current maturities	153,100	153,464	149,628
6.50% notes due 2016 [Member]
Long-term Debt
Total long-term debt including current maturities	516,701	518,293	500,000
6.40% notes due 2017 [Member]
Long-term Debt
Total long-term debt including current maturities	349,878	349,869	349,861
7.00% notes due 2018 [Member]
Long-term Debt
Total long-term debt including current maturities	399,711	399,693	399,675
10.375% notes due 2018 [Member]
Long-term Debt
Total long-term debt including current maturities	248,599	248,526	248,457
7.50% notes due 2021 [Member]
Long-term Debt
Total long-term debt including current maturities	600,000	600,000	600,000
7.15% notes due 2037 [Member]
Long-term Debt
Total long-term debt including current maturities	239,549	239,545	239,717
Medium-term notes [Member]
Long-term Debt
Total long-term debt including current maturities	16,000	16,000	21,000
Industrial revenue bonds [Member]
Long-term Debt
Total long-term debt including current maturities	14,000	14,000	14,000
Other notes [Member]
Long-term Debt
Total long-term debt including current maturities	1,084	1,189	1,349
Fair value adjustments [Member]
Long-term Debt
Total long-term debt including current maturities	$ 0	$ 0	$ (7,419)