Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs included in cost of product sold (2)	$ 284,215	$ 330,755	$ 888,743	$ 947,120
Amortization of prior service costs included in selling, general and administrative expenses (2)	12,940	12,163	36,701	37,086
Gain on sale of Decker Mine interest	0	(74,262)	0	(74,262)
Total before tax	(6,763)	(131,744)	61,348	(103,442)
Post retirement medical plan | Amount Reclassified from Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs included in cost of product sold (2)	264	209	792	626
Amortization of prior service costs included in selling, general and administrative expenses (2)	49	38	147	115
Gain on sale of Decker Mine interest		3,183		3,183
Total before tax	313	3,430	939	3,924
Tax Benefit	(116)	(1,235)	(347)	(1,413)
Amounts reclassified from AOCI	$ 197	$ 2,195	$ 592	$ 2,511