Summary of Carrying and Fair Values of Long Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
6 5/8% Senior Notes Due 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value	$ 249,715	$ 249,674
Senior Notes, Fair Value	252,160	254,890
6 1/2% Senior Notes Due 2029
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value	100,000	100,000
Senior Notes, Fair Value	115,575	108,834
6 3/4% Senior Notes Due 2032
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value	250,000	250,000
Senior Notes, Fair Value	280,583	263,988
2035 GO Zone 6 1/2% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value	89,000	89,000
Senior Notes, Fair Value	89,619	93,090
2035 IKE Zone 6 1/2% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value	65,000	65,000
Senior Notes, Fair Value	65,452	67,987
Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, Carrying Value	10,889	10,889
Loans, Fair Value	$ 10,889	$ 10,889