Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
ASSETS
Cash and cash equivalents	$ 468,532	$ 260,818	$ 116,601	$ 128,430
Trade receivables, net	91,057	149,116
Other receivables	127,037	132,647
Intercompany receivables	0	0
Intercompany loans receivable		0
Inventories	201,598	312,647
Deferred income taxes	16,819	37,067
Prepaid expenses	46,190	39,022
Other current assets	19,542	18,031
Total current assets	970,775	949,348
Mineral interests, net	2,836,801	2,905,002
Property, plant and equipment, net	1,466,297	1,637,552
Deferred income taxes		0
Investment in subsidiaries	0	0
Other long-term assets	112,256	98,958
Total assets	5,386,129	5,590,860	5,768,420
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	12,327	9,210
Accounts payable	38,980	92,712
Accrued expenses	125,318	133,870
Intercompany payables	0	0
Intercompany loans payable		0
Pension and other postretirement benefits obligation	29,032	37,125
Other current liabilities	215,952	206,984
Total current liabilities	421,609	479,901
Long-term debt	3,123,643	2,769,622
Pension and other postretirement benefits obligation	641,231	572,768
Deferred income taxes	730,685	822,867
Other long-term liabilities	187,380	193,008
Total liabilities	5,104,548	4,838,166
Total stockholders' equity	281,581	752,694	1,010,571	2,136,517
Total liabilities and stockholders' equity	5,386,129	5,590,860
Parent (Issuer)
ASSETS
Cash and cash equivalents	421,533	234,150	83,833	99,086
Trade receivables, net	0	0
Other receivables	123,659	113,936
Intercompany receivables	0	0
Intercompany loans receivable		63,549
Inventories	0	0
Deferred income taxes	15,986	23,957
Prepaid expenses	3,424	2,245
Other current assets	10,363	15,257
Total current assets	574,965	453,094
Mineral interests, net	0	0
Property, plant and equipment, net	7,558	7,248
Deferred income taxes		3,049
Investment in subsidiaries	3,233,399	4,409,683
Other long-term assets	87,928	73,564
Total assets	3,903,850	4,946,638
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	0	0
Accounts payable	2,296	5,604
Accrued expenses	43,088	34,551
Intercompany payables	221,070	87,904
Intercompany loans payable		1,104,282
Pension and other postretirement benefits obligation	95	94
Other current liabilities	168,444	164,364
Total current liabilities	434,993	1,396,799
Long-term debt	3,117,886	2,763,957
Pension and other postretirement benefits obligation	10,502	263
Deferred income taxes	23,766	0
Other long-term liabilities	35,122	32,925
Total liabilities	3,622,269	4,193,944
Total stockholders' equity	281,581	752,694
Total liabilities and stockholders' equity	3,903,850	4,946,638
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	1,117	1,620	8,158	8,295
Trade receivables, net	88,959	92,027
Other receivables	2,193	3,157
Intercompany receivables	206,118	29,735
Intercompany loans receivable		1,104,282
Inventories	110,882	176,981
Deferred income taxes	0	12,716
Prepaid expenses	40,044	34,317
Other current assets	7,029	440
Total current assets	456,342	1,455,275
Mineral interests, net	135,377	157,812
Property, plant and equipment, net	726,478	824,729
Deferred income taxes		0
Investment in subsidiaries	6,459	6,401
Other long-term assets	17,857	13,186
Total assets	1,342,513	2,457,403
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	7,209	1,424
Accounts payable	30,697	68,370
Accrued expenses	60,762	57,036
Intercompany payables	0	0
Intercompany loans payable		0
Pension and other postretirement benefits obligation	28,937	37,031
Other current liabilities	27,172	22,443
Total current liabilities	154,777	186,304
Long-term debt	5,704	22
Pension and other postretirement benefits obligation	630,729	572,505
Deferred income taxes	0	24,079
Other long-term liabilities	96,599	87,944
Total liabilities	887,809	870,854
Total stockholders' equity	454,704	1,586,549
Total liabilities and stockholders' equity	1,342,513	2,457,403
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	45,882	25,048	24,610	21,049
Trade receivables, net	2,098	57,089
Other receivables	1,185	15,554
Intercompany receivables	14,952	58,169
Intercompany loans receivable		0
Inventories	90,716	135,666
Deferred income taxes	833	394
Prepaid expenses	2,722	2,460
Other current assets	2,150	2,334
Total current assets	160,538	296,714
Mineral interests, net	2,701,424	2,747,190
Property, plant and equipment, net	732,261	805,575
Deferred income taxes		0
Investment in subsidiaries	0	0
Other long-term assets	6,471	12,208
Total assets	3,600,694	3,861,687
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	5,118	7,786
Accounts payable	5,987	18,738
Accrued expenses	21,468	42,283
Intercompany payables	0	0
Intercompany loans payable		63,549
Pension and other postretirement benefits obligation	0	0
Other current liabilities	20,336	20,177
Total current liabilities	52,909	152,533
Long-term debt	53	5,643
Pension and other postretirement benefits obligation	0	0
Deferred income taxes	706,919	801,837
Other long-term liabilities	55,659	72,139
Total liabilities	815,540	1,032,152
Total stockholders' equity	2,785,154	2,829,535
Total liabilities and stockholders' equity	3,600,694	3,861,687
Eliminations
ASSETS
Cash and cash equivalents	0	0	0	0
Trade receivables, net	0	0
Other receivables	0	0
Intercompany receivables	(221,070)	(87,904)
Intercompany loans receivable		(1,167,831)
Inventories	0	0
Deferred income taxes	0	0
Prepaid expenses	0	0
Other current assets	0	0
Total current assets	(221,070)	(1,255,735)
Mineral interests, net	0	0
Property, plant and equipment, net	0	0
Deferred income taxes		(3,049)
Investment in subsidiaries	(3,239,858)	(4,416,084)
Other long-term assets	0	0
Total assets	(3,460,928)	(5,674,868)
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	0	0
Accounts payable	0	0
Accrued expenses	0	0
Intercompany payables	(221,070)	(87,904)
Intercompany loans payable		(1,167,831)
Pension and other postretirement benefits obligation	0	0
Other current liabilities	0	0
Total current liabilities	(221,070)	(1,255,735)
Long-term debt	0	0
Pension and other postretirement benefits obligation	0	0
Deferred income taxes	0	(3,049)
Other long-term liabilities	0	0
Total liabilities	(221,070)	(1,258,784)
Total stockholders' equity	(3,239,858)	(4,416,084)
Total liabilities and stockholders' equity	$ (3,460,928)	$ (5,674,868)