Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
ASSETS
Cash and cash equivalents	$ 235,791	$ 116,601	$ 138,200	$ 128,430
Receivables, net	315,296	256,967
Inventories	302,293	306,018
Deferred income taxes	55,629	58,526
Prepaid expenses	47,441	53,776
Other current assets	23,285	23,928
Total current assets	979,735	815,816
Mineral interests, net	2,927,591	2,965,557
Property, plant and equipment, net	1,691,637	1,732,131
Deferred income taxes	185,387	160,422
Other long-term assets	99,092	94,494
Total assets	5,883,442	5,768,420
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	17,398	18,793
Accounts payable	133,040	114,913
Accrued expenses	192,913	184,875
Accumulated postretirement benefits obligation	29,637	29,200
Other current liabilities	207,328	206,473
Total current liabilities	580,316	554,254
Long-term debt	2,593,584	2,397,372
Deferred income taxes	883,659	921,687
Accumulated postretirement benefits obligation	636,129	633,264
Other long-term liabilities	244,084	251,272
Total liabilities	4,937,772	4,757,849
Total stockholders' equity	945,670	1,010,571
Total liabilities and stockholders' equity	5,883,442	5,768,420
Parent (Issuer)
ASSETS
Cash and cash equivalents	200,185	83,833	126,517	99,086
Receivables, net	79,729	64,106
Intercompany receivables		153,933
Intercompany notes receivable	143,263	118,079
Deferred income taxes	36,717	39,375
Prepaid expenses	1,374	1,869
Other current assets	18,146	17,559
Total current assets	479,414	478,754
Property, plant and equipment, net	8,276	8,448
Deferred income taxes	78,056	52,363
Investment in subsidiaries	3,815,313	3,530,094
Other long-term assets	76,091	71,622
Total assets	4,457,150	4,141,281
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	11,588	5,128
Accrued expenses	100,956	27,197
Intercompany payable	613,761	507,519
Accumulated postretirement benefits obligation	131	131
Other current liabilities	157,201	157,044
Total current liabilities	883,637	697,019
Long-term debt	2,581,327	2,381,255
Accumulated postretirement benefits obligation	435	452
Other long-term liabilities	46,081	51,984
Total liabilities	3,511,480	3,130,710
Total stockholders' equity	945,670	1,010,571
Total liabilities and stockholders' equity	4,457,150	4,141,281
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	158	61	109	79
Receivables, net	122,336	113,182
Intercompany receivables	538,342	507,519
Inventories	118,744	131,893
Deferred income taxes	17,687	17,687
Prepaid expenses	40,861	45,327
Other current assets	1,128	1,109
Total current assets	839,256	816,778
Mineral interests, net	17,015	18,475
Property, plant and equipment, net	779,101	790,900
Deferred income taxes	112,560	112,560
Other long-term assets	9,674	9,375
Total assets	1,757,606	1,748,088
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	9,277	10,196
Accounts payable	80,694	78,260
Accrued expenses	87,102	83,155
Accumulated postretirement benefits obligation	29,506	29,069
Other current liabilities	23,619	24,389
Total current liabilities	230,198	225,069
Long-term debt	115	1,784
Accumulated postretirement benefits obligation	635,694	632,812
Other long-term liabilities	128,839	128,593
Total liabilities	994,846	988,258
Total stockholders' equity	762,760	759,830
Total liabilities and stockholders' equity	1,757,606	1,748,088
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	35,448	32,707	11,574	29,265
Receivables, net	113,231	79,679
Intercompany receivables	75,419
Inventories	183,549	174,125
Deferred income taxes	1,225	1,464
Prepaid expenses	5,206	6,580
Other current assets	4,011	5,260
Total current assets	418,089	299,815
Mineral interests, net	2,910,576	2,947,082
Property, plant and equipment, net	904,260	932,783
Deferred income taxes	(5,229)	(4,501)
Other long-term assets	13,327	13,497
Total assets	4,241,023	4,188,676
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	8,121	8,597
Accounts payable	40,758	31,525
Accrued expenses	4,855	74,523
Intercompany payable		153,933
Intercompany notes payable	143,263	118,079
Other current liabilities	26,508	25,040
Total current liabilities	223,505	411,697
Long-term debt	12,142	14,333
Deferred income taxes	883,659	921,687
Other long-term liabilities	69,164	70,695
Total liabilities	1,188,470	1,418,412
Total stockholders' equity	3,052,553	2,770,264
Total liabilities and stockholders' equity	4,241,023	4,188,676
Eliminations
ASSETS
Intercompany receivables	(613,761)	(661,452)
Intercompany notes receivable	(143,263)	(118,079)
Total current assets	(757,024)	(779,531)
Investment in subsidiaries	(3,815,313)	(3,530,094)
Total assets	(4,572,337)	(4,309,625)
LIABILITIES AND STOCKHOLDERS' EQUITY
Intercompany payable	(613,761)	(661,452)
Intercompany notes payable	(143,263)	(118,079)
Total current liabilities	(757,024)	(779,531)
Total liabilities	(757,024)	(779,531)
Total stockholders' equity	(3,815,313)	(3,530,094)
Total liabilities and stockholders' equity	$ (4,572,337)	$ (4,309,625)