The Effect of Derivative Instruments on the Consolidated Statement of Income (Details) (Price Swaps, Derivative in Cash Flow Hedging Relationship, Natural Gas, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 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	$ 60,566	$ 78,463	$ 60,566	$ 78,463	$ 71,674	$ 76,761	$ 132,731	$ 151,780
Amount of Gain (Loss) Recognized in OCI on Derivative	13,246	(6,459)	40,400	80,280
Outside Sales
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income	24,354	47,809	56,595	153,597
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 2,592	$ 1,732	$ (120)	$ 1,778