Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
ASSETS
Cash and cash equivalents	$ 170,878	$ 116,601	$ 128,680	$ 128,430
Receivables, net	254,256	256,967
Inventories	339,584	306,018
Deferred income taxes	55,318	58,526
Prepaid expenses	60,151	53,776
Other current assets	23,754	23,928
Total current assets	903,941	815,816
Mineral interests, net	2,914,825	2,965,557
Property, plant and equipment, net	1,657,123	1,732,131
Deferred income taxes	161,215	160,422
Other long-term assets	94,752	94,494
Total assets	5,731,856	5,768,420
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	14,919	18,793
Accounts payable	123,540	114,913
Accrued expenses	135,772	184,875
Accumulated postretirement benefits obligation	30,074	29,200
Other current liabilities	202,300	206,473
Total current liabilities	506,605	554,254
Long-term debt	2,591,181	2,397,372
Accumulated postretirement benefits obligation	638,713	633,264
Deferred income taxes	848,748	921,687
Other long-term liabilities	237,320	251,272
Total liabilities	4,822,567	4,757,849
Total stockholders' equity	909,289	1,010,571
Total liabilities and stockholders' equity	5,731,856	5,768,420
Parent (Issuer)
ASSETS
Cash and cash equivalents	136,634	83,833	160,932	99,086
Receivables, net	96,820	64,106
Intercompany receivables		153,933
Intercompany loans receivable	152,755	118,079
Deferred income taxes	39,656	39,375
Prepaid expenses	23,635	1,869
Other current assets	19,406	17,559
Total current assets	468,906	478,754
Property, plant and equipment, net	7,910	8,448
Deferred income taxes	53,157	52,363
Investment in subsidiaries	4,537,487	3,530,094
Other long-term assets	72,142	71,622
Total assets	5,139,602	4,141,281
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	26,164	5,128
Accrued expenses	31,526	27,197
Intercompany payables	1,388,447	507,519
Accumulated postretirement benefits obligation	131	131
Other current liabilities	159,413	157,044
Total current liabilities	1,605,681	697,019
Long-term debt	2,581,402	2,381,255
Accumulated postretirement benefits obligation	344	452
Other long-term liabilities	42,886	51,984
Total liabilities	4,230,313	3,130,710
Total stockholders' equity	909,289	1,010,571
Total liabilities and stockholders' equity	5,139,602	4,141,281
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents		61	94,085	79
Receivables, net	91,498	113,182
Intercompany receivables	1,319,675	507,519
Inventories	159,887	131,893
Deferred income taxes	14,914	17,687
Prepaid expenses	29,891	45,327
Other current assets	694	1,109
Total current assets	1,616,559	816,778
Mineral interests, net	9,621	18,475
Property, plant and equipment, net	772,302	790,900
Deferred income taxes	112,560	112,560
Other long-term assets	10,892	9,375
Total assets	2,521,934	1,748,088
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	6,734	10,196
Accounts payable	72,887	78,260
Accrued expenses	64,509	83,155
Accumulated postretirement benefits obligation	29,943	29,069
Other current liabilities	23,516	24,389
Total current liabilities	197,589	225,069
Long-term debt	345	1,784
Accumulated postretirement benefits obligation	637,800	632,812
Other long-term liabilities	125,843	128,593
Total liabilities	961,577	988,258
Total stockholders' equity	1,560,357	759,830
Total liabilities and stockholders' equity	2,521,934	1,748,088
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	34,244	32,707	(66,337)	29,265
Receivables, net	65,938	79,679
Intercompany receivables	68,772
Inventories	179,697	174,125
Deferred income taxes	748	1,464
Prepaid expenses	6,625	6,580
Other current assets	3,654	5,260
Total current assets	359,678	299,815
Mineral interests, net	2,905,204	2,947,082
Property, plant and equipment, net	876,911	932,783
Deferred income taxes	(4,502)	(4,501)
Other long-term assets	11,718	13,497
Total assets	4,149,009	4,188,676
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	8,185	8,597
Accounts payable	24,489	31,525
Accrued expenses	39,737	74,523
Intercompany payables		153,933
Intercompany loans payable	152,755	118,079
Other current liabilities	19,371	25,040
Total current liabilities	244,537	411,697
Long-term debt	9,434	14,333
Accumulated postretirement benefits obligation	569
Deferred income taxes	848,748	921,687
Other long-term liabilities	68,591	70,695
Total liabilities	1,171,879	1,418,412
Total stockholders' equity	2,977,130	2,770,264
Total liabilities and stockholders' equity	4,149,009	4,188,676
Eliminations
ASSETS
Intercompany receivables	(1,388,447)	(661,452)
Intercompany loans receivable	(152,755)	(118,079)
Total current assets	(1,541,202)	(779,531)
Investment in subsidiaries	(4,537,487)	(3,530,094)
Total assets	(6,078,689)	(4,309,625)
LIABILITIES AND STOCKHOLDERS' EQUITY
Intercompany payables	(1,388,447)	(661,452)
Intercompany loans payable	(152,755)	(118,079)
Total current liabilities	(1,541,202)	(779,531)
Total liabilities	(1,541,202)	(779,531)
Total stockholders' equity	(4,537,487)	(3,530,094)
Total liabilities and stockholders' equity	$ (6,078,689)	$ (4,309,625)