The Effect of Derivative Instruments on the Consolidated Statement of Income (Details) (Swap [Member], Cash Flow Hedging [Member], Natural Gas [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Swap [Member] | Cash Flow Hedging [Member] | Natural Gas [Member]
Derivative [Line Items]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	$ 6,574	$ 71,674	$ 6,574	$ 71,674	$ 11,841	$ 42,493	$ 35,453	$ 76,761
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	(12,218)	45,749	(59,183)	27,154
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(6,951)	9,528	(23,264)	32,241
Derivative, Gain (Loss) on Derivative, Net	$ 508	$ (3,753)	$ 863	$ (2,712)