Fair Value Measurements (Summary of Carrying and Fair Values of Long Term Debt) (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	1 Months Ended	9 Months Ended					0 Months Ended	9 Months Ended					9 Months Ended					9 Months Ended					9 Months Ended					9 Months Ended					9 Months Ended
	Jul. 30, 2012 6 5/8% Senior Notes Due 2016	Jul. 31, 2012 6 5/8% Senior Notes Due 2016	Sep. 30, 2012 6 5/8% Senior Notes Due 2016	Sep. 30, 2012 6 5/8% Senior Notes Due 2016 Carrying Value	Dec. 31, 2011 6 5/8% Senior Notes Due 2016 Carrying Value	Sep. 30, 2012 6 5/8% Senior Notes Due 2016 Fair Value	Dec. 31, 2011 6 5/8% Senior Notes Due 2016 Fair Value	Jul. 17, 2012 3.60% Senior Notes Due 2022	Sep. 30, 2012 3.60% Senior Notes Due 2022	Sep. 30, 2012 3.60% Senior Notes Due 2022 Carrying Value	Dec. 31, 2011 3.60% Senior Notes Due 2022 Carrying Value	Sep. 30, 2012 3.60% Senior Notes Due 2022 Fair Value	Dec. 31, 2011 3.60% Senior Notes Due 2022 Fair Value	Sep. 30, 2012 6 1/2% Senior Notes Due 2029	Sep. 30, 2012 6 1/2% Senior Notes Due 2029 Carrying Value	Dec. 31, 2011 6 1/2% Senior Notes Due 2029 Carrying Value	Sep. 30, 2012 6 1/2% Senior Notes Due 2029 Fair Value	Dec. 31, 2011 6 1/2% Senior Notes Due 2029 Fair Value	Sep. 30, 2012 6 3/4% Senior Notes Due 2032	Sep. 30, 2012 6 3/4% Senior Notes Due 2032 Carrying Value	Dec. 31, 2011 6 3/4% Senior Notes Due 2032 Carrying Value	Sep. 30, 2012 6 3/4% Senior Notes Due 2032 Fair Value	Dec. 31, 2011 6 3/4% Senior Notes Due 2032 Fair Value	Sep. 30, 2012 2035 GO Zone 6 1/2% Notes	Sep. 30, 2012 2035 GO Zone 6 1/2% Notes Carrying Value	Dec. 31, 2011 2035 GO Zone 6 1/2% Notes Carrying Value	Sep. 30, 2012 2035 GO Zone 6 1/2% Notes Fair Value	Dec. 31, 2011 2035 GO Zone 6 1/2% Notes Fair Value	Sep. 30, 2012 2035 IKE Zone 6 1/2% Notes	Sep. 30, 2012 2035 IKE Zone 6 1/2% Notes Carrying Value	Dec. 31, 2011 2035 IKE Zone 6 1/2% Notes Carrying Value	Sep. 30, 2012 2035 IKE Zone 6 1/2% Notes Fair Value	Dec. 31, 2011 2035 IKE Zone 6 1/2% Notes Fair Value	Sep. 30, 2012 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027	Sep. 30, 2012 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Carrying Value	Dec. 31, 2011 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Carrying Value	Sep. 30, 2012 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Fair Value	Dec. 31, 2011 Loan Related To Tax-Exempt Waste Disposal Revenue Bonds Due 2027 Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2016	2016	2016					2022	2022					2029					2032					2035					2035					2027
Stated interest rate	6.625%		6.625%					3.60%	3.60%					6.50%					6.75%					6.50%					6.50%
Senior notes				$ 0	$ 249,674	$ 0	$ 254,890			$ 248,842	$ 0	$ 253,738	$ 0		$ 100,000	$ 100,000	$ 115,619	$ 108,834		$ 250,000	$ 250,000	$ 288,515	$ 263,988		$ 89,000	$ 89,000	$ 103,031	$ 93,090		$ 65,000	$ 65,000	$ 75,247	$ 67,987
Loans																																			$ 10,889	$ 10,889	$ 10,889	$ 10,889