Long-Term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Deferred charges and other assets, net	$ 282,695	$ 167,417
Principal Amount	764,889	764,889
Unamortized Discount and Debt Issuance Costs,	(6,436)	(6,741)
Long-term debt, net	$ 758,453	758,148
3.60% Senior Notes Due 2022
Debt Instrument [Line Items]
Maturity date	2022
Stated interest rate	3.60%
6 1/2% Senior Notes Due 2029
Debt Instrument [Line Items]
Maturity date	2029
Stated interest rate	6.50%
6 3/4% Senior Notes Due 2032
Debt Instrument [Line Items]
Maturity date	2032
Stated interest rate	6.75%
2035 GO Zone 6 1/2% Notes
Debt Instrument [Line Items]
Maturity date	2035
Stated interest rate	6.50%
2035 IKE Zone 6 1/2% Notes
Debt Instrument [Line Items]
Maturity date	2035
Stated interest rate	6.50%
Loan related to tax-exempt waste disposal revenue bonds due 2027
Debt Instrument [Line Items]
Maturity date	2027
Loan related to tax-exempt waste disposal revenue bonds due 2027 | Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	$ 10,889	10,889
Unamortized Discount and Debt Issuance Costs,	0	0
Long-term debt, net	10,889	10,889
3.60% Senior Notes Due 2022 | Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	250,000	250,000
Unamortized Discount and Debt Issuance Costs,	(2,061)	(2,232)
Long-term debt, net	247,939	247,768
6 1/2% Senior Notes Due 2029 | Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	100,000	100,000
Unamortized Discount and Debt Issuance Costs,	(952)	(989)
Long-term debt, net	99,048	99,011
6 3/4% Senior Notes Due 2032 | Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	250,000	250,000
Unamortized Discount and Debt Issuance Costs,	(1,943)	(2,002)
Long-term debt, net	248,057	247,998
2035 GO Zone 6 1/2% Notes | Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	89,000	89,000
Unamortized Discount and Debt Issuance Costs,	(862)	(884)
Long-term debt, net	88,138	88,116
2035 IKE Zone 6 1/2% Notes | Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	65,000	65,000
Unamortized Discount and Debt Issuance Costs,	(618)	(634)
Long-term debt, net	$ 64,382	64,366
Previously reported [Member]
Debt Instrument [Line Items]
Deferred charges and other assets, net		173,384
Long-term debt, net		$ 764,115