Fair Value Measurements (Summary of Carrying and Fair Values of Long Term Debt) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended					3 Months Ended					3 Months Ended					3 Months Ended					3 Months Ended					3 Months Ended
	Sep. 30, 2013 3.60% Senior Notes Due 2022	Sep. 30, 2013 3.60% Senior Notes Due 2022 Carrying Value	Dec. 31, 2012 3.60% Senior Notes Due 2022 Carrying Value	Sep. 30, 2013 3.60% Senior Notes Due 2022 Fair Value	Dec. 31, 2012 3.60% Senior Notes Due 2022 Fair Value	Sep. 30, 2013 6 1/2% Senior Notes Due 2029	Sep. 30, 2013 6 1/2% Senior Notes Due 2029 Carrying Value	Dec. 31, 2012 6 1/2% Senior Notes Due 2029 Carrying Value	Sep. 30, 2013 6 1/2% Senior Notes Due 2029 Fair Value	Dec. 31, 2012 6 1/2% Senior Notes Due 2029 Fair Value	Sep. 30, 2013 6 3/4% Senior Notes Due 2032	Sep. 30, 2013 6 3/4% Senior Notes Due 2032 Carrying Value	Dec. 31, 2012 6 3/4% Senior Notes Due 2032 Carrying Value	Sep. 30, 2013 6 3/4% Senior Notes Due 2032 Fair Value	Dec. 31, 2012 6 3/4% Senior Notes Due 2032 Fair Value	Sep. 30, 2013 2035 GO Zone 6 1/2% Notes	Sep. 30, 2013 2035 GO Zone 6 1/2% Notes Carrying Value	Dec. 31, 2012 2035 GO Zone 6 1/2% Notes Carrying Value	Sep. 30, 2013 2035 GO Zone 6 1/2% Notes Fair Value	Dec. 31, 2012 2035 GO Zone 6 1/2% Notes Fair Value	Sep. 30, 2013 2035 IKE Zone 6 1/2% Notes	Sep. 30, 2013 2035 IKE Zone 6 1/2% Notes Carrying Value	Dec. 31, 2012 2035 IKE Zone 6 1/2% Notes Carrying Value	Sep. 30, 2013 2035 IKE Zone 6 1/2% Notes Fair Value	Dec. 31, 2012 2035 IKE Zone 6 1/2% Notes Fair Value	Sep. 30, 2013 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027	Sep. 30, 2013 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Carrying Value	Dec. 31, 2012 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Carrying Value	Sep. 30, 2013 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Fair Value	Dec. 31, 2012 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2022					2029					2032					2035					2035					2027
Stated interest rate	3.60%					6.50%					6.75%					6.50%					6.50%
Senior notes		$ 248,960	$ 248,872	$ 238,228	$ 251,125		$ 100,000	$ 100,000	$ 107,800	$ 119,738		$ 250,000	$ 250,000	$ 267,465	$ 283,168		$ 89,000	$ 89,000	$ 94,209	$ 102,095		$ 65,000	$ 65,000	$ 68,804	$ 74,564
Loans																											$ 10,889	$ 10,889	$ 10,889	$ 10,889