Summary of Debt (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Long Term Debt
Total long-term debt including current maturities	$ 2,666,024		$ 2,677,003		$ 2,814,458
Less current maturities	140,604		150,602		144,706
Total long-term debt	2,525,420		2,526,401		2,669,752
Estimated fair value of long-term debt	2,851,237		2,766,835		2,864,657
Line of Credit
Long Term Debt
Total long-term debt including current maturities	0		0		0
5.60% notes due 2012
Long Term Debt
Total long-term debt including current maturities	0	[1]	0	[1]	134,521	[1]
6.30% notes due 2013
Long Term Debt
Total long-term debt including current maturities	140,430	[2]	140,413	[2]	140,367	[2]
10.125% notes due 2015
Long Term Debt
Total long-term debt including current maturities	152,520	[3]	152,718	[3]	153,284	[3]
6.50% notes due 2016
Long Term Debt
Total long-term debt including current maturities	514,221	[4]	515,060	[4]	517,503	[4]
6.40% notes due 2017
Long Term Debt
Total long-term debt including current maturities	349,892	[5]	349,888	[5]	349,874	[5]
7.00% notes due 2018
Long Term Debt
Total long-term debt including current maturities	399,741	[6]	399,731	[6]	399,702	[6]
10.375% notes due 2018
Long Term Debt
Total long-term debt including current maturities	248,716	[7]	248,676	[7]	248,562	[7]
7.50% notes due 2021
Long Term Debt
Total long-term debt including current maturities	600,000	[8]	600,000	[8]	600,000	[8]
7.15% notes due 2037
Long Term Debt
Total long-term debt including current maturities	239,555	[9]	239,553	[9]	239,547	[9]
Medium-term Notes
Long Term Debt
Total long-term debt including current maturities	6,000		16,000		16,000
Municipal Bonds
Long Term Debt
Total long-term debt including current maturities	14,000		14,000		14,000
Other Notes
Long Term Debt
Total long-term debt including current maturities	$ 949		$ 964		$ 1,098

[1] Includes decreases for unamortized discounts, as follows: March 31, 2012 - $36 thousand.

[2] Includes decreases for unamortized discounts, as follows: March 31, 2013 - $14 thousand, December 31, 2012 - $30 thousand and March 31, 2012 - $77 thousand. The effective interest rate for these notes is 7.48%.

[3] Includes an increase for the unamortized portion of the deferred gain realized upon the August 2011 settlement of interest rate swaps, as follows: March 31, 2013 - $2,766 thousand, December 31, 2012 - $2,983 thousand and March 31, 2012 - $3,604 thousand. Additionally, includes decreases for unamortized discounts, as follows: March 31, 2013 - $246 thousand, December 31, 2012 - $265 thousand and March 31, 2012 - $320 thousand. The effective interest rate for these notes is 9.59%.

[4] Includes an increase for the unamortized portion of the deferred gain realized upon the August 2011 settlement of interest rate swaps, as follows: March 31, 2013 - $14,221 thousand, December 31, 2012 - $15,060 thousand and March 31, 2012 - $17,503 thousand. The effective interest rate for these notes is 6.02%.

[5] Includes decreases for unamortized discounts, as follows: March 31, 2013 - $108 thousand, December 31, 2012 - $112 thousand and March 31, 2012 - $126 thousand. The effective interest rate for these notes is 7.41%.

[6] Includes decreases for unamortized discounts, as follows: March 31, 2013 - $259 thousand, December 31, 2012 - $269 thousand and March 31, 2012 - $298 thousand. The effective interest rate for these notes is 7.87%.

[7] Includes decreases for unamortized discounts, as follows: March 31, 2013 - $1,284 thousand, December 31, 2012 - $1,324 thousand and March 31, 2012 - $1,438 thousand. The effective interest rate for these notes is 10.62%.

[8] The effective interest rate for these notes is 7.75%.

[9] Includes decreases for unamortized discounts, as follows: March 31, 2013 - $633 thousand, December 31, 2012 - $635 thousand and March 31, 2012 - $641 thousand. The effective interest rate for these notes is 8.05%.