Significant Accounting Policy Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	$ 0	$ 43,470
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	(68,481)	(123,105)	$ (28,753)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	25,011	45,054	10,465
Defined Benefit Plan, Actuarial Net (Gains) Losses	23,857	119,222	46,155
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	0	5	(36,182)
Defined Benefit Plan, Net Periodic Benefit Cost	22,196	19,053	29,095
Net periodic (benefit) cost	45,463	(198,713)	16,687
Actuarially Determined Long-Term Liability Tax Expense	(16,959)	74,687	(6,139)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(590)	(336,993)	(22,381)
Actuarially Long-Term Liabilities Net of Tax	28,504	(124,026)	10,548
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	(392,556)	(359,068)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(392,556)	(315,598)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(61,730)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(14,966)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	262
Commodity Contract [Member] | Cash Flow Hedging [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion	(43,470)	$ (78,051)	$ (18,288)
Postretirement Benefit Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(61,730)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	28,504
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	262
Cash Flow Hedging [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(43,470)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	$ 0