CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income	$ 568,032	$ 204,554	$ 253,562
Cash flow hedges:
Total cash flow hedges	(12,178)	(32,018)	35,864
Pension and postretirement plans:
Amortization of net benefit obligation at transition, net of tax of $8, $112 and $100, respectively	14	207	186
Amortization of prior service cost, net of tax of $87, $90 and $119, respectively	161	167	221
Amortization of net loss, net of tax of $7,676, $4,472 and $1,676, respectively	14,256	8,306	3,113
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Reclassification Adjustments, Net of Tax	(5,056)	11,582	(17,443)
Total pension and postretirement plans	9,375	20,262	(13,923)
Comprehensive Income	565,229	192,798	275,503
Commodity Contract
Cash flow hedges:
Reclassification adjustment for derivative instruments, net of tax	37	(10,866)	66,438
Reclassification adjustment for derivative instruments, net of tax	(13,399)	(22,124)	(29,359)
Interest Rate Swap
Cash flow hedges:
Reclassification adjustment for derivative instruments, net of tax	(298)	(148)	(2,281)
Reclassification adjustment for derivative instruments, net of tax	$ 1,482	$ 1,120	$ 1,066