Debt (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Short-term Borrowings
Bank line of credit	$ 0	$ 0	$ 300,000
Total short-term borrowings	0	0	300,000
Long-term Debt
Total long-term debt including current maturities	2,814,458	2,815,439	2,432,834
Less current maturities of long-term debt	144,706	134,762	5,238
Total long-term debt	2,669,752	2,680,677	2,427,596
Estimated fair value of total long-term debt	2,864,657	2,796,504	2,544,368
6.30% notes due 2013 [Member]
Long-term Debt
Total long-term debt including current maturities	140,367	140,352	249,754
Bank line of credit [Member]
Long-term Debt
Total long-term debt including current maturities	0	0	0
5.60% notes due 2012 [Member]
Long-term Debt
Total long-term debt including current maturities	134,521	134,508	299,801
Floating-rate term loan due 2015 [Member]
Long-term Debt
Total long-term debt including current maturities	0	0	450,000
10.125% notes due 2015 [Member]
Long-term Debt
Total long-term debt including current maturities	153,284	153,464	149,612
6.50% notes due 2016 [Member]
Long-term Debt
Total long-term debt including current maturities	517,503	518,293	0
6.40% notes due 2017 [Member]
Long-term Debt
Total long-term debt including current maturities	349,874	349,869	349,856
7.00% notes due 2018 [Member]
Long-term Debt
Total long-term debt including current maturities	399,702	399,693	399,666
10.375% notes due 2018 [Member]
Long-term Debt
Total long-term debt including current maturities	248,562	248,526	248,424
7.50% notes due 2021 [Member]
Long-term Debt
Total long-term debt including current maturities	600,000	600,000	0
7.15% notes due 2037 [Member]
Long-term Debt
Total long-term debt including current maturities	239,547	239,545	249,326
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	$ 1,098	$ 1,189	$ 1,395