Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Gains (losses) on cash flow hedges:
Income tax expense	$ 130,338	$ (16,055)	$ 200,319	$ (23,287)
Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Net of tax	(474)	2,577	(2,901)	586
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Total	0	5,745	0	13,501
Income tax expense	0	(2,191)	0	(5,199)
Net of tax	0	3,554	0	8,302
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Total	(730)	(1,502)	(4,462)	(11,871)
Income tax expense	256	525	1,561	4,155
Net of tax	(474)	(977)	(2,901)	(7,716)
Pension and postretirement plans:
Transition obligation	0	2	0	(17)
Prior service cost	0	(74)	0	(222)
Actuarial losses	(730)	(1,430)	(4,462)	(11,632)
Commodity contracts | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Reclassification Adjustment from AOCI on Derivatives	0	5,992	0	15,781
Interest rate swap | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Reclassification Adjustment from AOCI on Derivatives	$ 0	$ (247)	$ 0	$ (2,280)