Schedule of Long Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term debt	$ 764,604	$ 764,563
6 5/8% Senior Notes Due 2016
Debt Instrument [Line Items]
Senior notes	249,715	249,674
6 1/2% Senior Notes Due 2029
Debt Instrument [Line Items]
Senior notes	100,000	100,000
6 3/4% Senior Notes Due 2032
Debt Instrument [Line Items]
Senior notes	250,000	250,000
2035 GO Zone 6 1/2% Notes
Debt Instrument [Line Items]
Senior notes	89,000	89,000
2035 IKE Zone 6 1/2% Notes
Debt Instrument [Line Items]
Senior notes	65,000	65,000
Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027
Debt Instrument [Line Items]
Non-current loan payable	$ 10,889	$ 10,889