Fair Value Measurements (Summary of Carrying and Fair Values of Long Term Debt) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
3.60% Senior Notes Due 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2022
Stated interest rate	3.60%
3.60% Senior Notes Due 2022 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	249,138	$ 249,108
3.60% Senior Notes Due 2022 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	253,240	248,630
6 1/2% Senior Notes Due 2029
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2029
Stated interest rate	6.50%
6 1/2% Senior Notes Due 2029 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	100,000	100,000
6 1/2% Senior Notes Due 2029 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	118,493	116,384
6 3/4% Senior Notes Due 2032
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2032
Stated interest rate	6.75%
6 3/4% Senior Notes Due 2032 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	250,000	250,000
6 3/4% Senior Notes Due 2032 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	280,285	285,545
2035 GO Zone 6 1/2% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2035
Stated interest rate	6.50%
2035 GO Zone 6 1/2% Notes | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	89,000	89,000
2035 GO Zone 6 1/2% Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	105,298	106,504
2035 IKE Zone 6 1/2% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2035
Stated interest rate	6.50%
2035 IKE Zone 6 1/2% Notes | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	65,000	65,000
2035 IKE Zone 6 1/2% Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	76,903	77,784
Loan related to tax-exempt waste disposal revenue bonds due 2027
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity	2027
Loan related to tax-exempt waste disposal revenue bonds due 2027 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	10,889	10,889
Loan related to tax-exempt waste disposal revenue bonds due 2027 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	10,889	$ 10,889