Fair Value Measurements (Schedule Of Carrying And Fair Values Of Long-Term Debt) (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	12 Months Ended	1 Months Ended
	Jul. 17, 2012	Dec. 31, 2014	Dec. 31, 1997	Dec. 31, 2013	Sep. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate		6.50%
3.60% Senior Notes Due 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value		$ 249,108		$ 248,990
Senior Notes, Fair Value		248,630		236,905
Stated interest rate	3.60%	3.60%		3.60%	3.60%
Maturity date	2022	2022
GO Zone 6 1/2% Senior Notes Due 2029 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value		100,000		100,000
Senior Notes, Fair Value		116,384		109,490
Stated interest rate		6.50%		6.50%
Maturity date		2029
GO Zone 6 3/4% Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value		250,000		250,000
Senior Notes, Fair Value		285,545		265,148
Stated interest rate		6.75%		6.75%
Maturity date		2032
GO Zone 6 1/2% Tax-Exempt Revenue Bonds Due 2035 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value		89,000		89,000
Senior Notes, Fair Value		106,504		94,606
Stated interest rate		6.50%		6.50%
Maturity date		2035
Ike Zone Six Point Five Percent Tax Exempt Revenue Bonds Due Twenty Thirty Five [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, Carrying Value		65,000		65,000
Senior Notes, Fair Value		77,784		69,094
Stated interest rate		6.50%		6.50%
Maturity date		2035
Waste Disposal Revenue Bonds Due 2027 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, Carrying Value		10,889		10,889
Loans, Fair Value		$ 10,889		$ 10,889
Maturity date		2027	December 2027