Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Gains (losses) on cash flow hedges:
Income tax expense	$ 61,280	$ 1,016	$ 69,981	$ (7,232)
Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Net of tax	(576)	1,398	(2,427)	(1,991)
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Total	0	4,147	0	7,756
Income tax expense	0	(1,623)	0	(3,008)
Net of tax	0	2,524	0	4,748
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Gains (losses) on cash flow hedges:
Total	(885)	(1,732)	(3,732)	(10,369)
Income tax expense	309	606	1,305	3,630
Net of tax	(576)	(1,126)	(2,427)	(6,739)
Pension and postretirement plans:
Transition obligation	0	(9)	0	(19)
Prior service cost	0	(74)	0	(148)
Actuarial losses	(885)	(1,649)	(3,732)	(10,202)
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	5,735	0	9,789
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	$ (1,588)	$ 0	$ (2,033)