The Effect of Derivative Instruments on the Consolidated Statement of Income (Details) (Price Swaps, Derivative in Cash Flow Hedging Relationship, Natural Gas, USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	$ 35,453,000	$ 179,915,000	$ 76,761,000	$ 151,780,000
Amount of Gain (Loss) Recognized in OCI on Derivative	(18,595,000)	76,076,000
Outside Sales
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income	22,713,000	47,941,000
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 1,041,000	$ (835,000)