Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
ASSETS
Cash and cash equivalents	$ 260,818	$ 116,601	$ 128,430	$ 293,410
Receivables, net	281,763	256,967
Inventories	312,647	306,018
Deferred income taxes	37,067	58,526
Prepaid expenses	39,022	53,776
Other current assets	18,031	23,928
Total current assets	949,348	815,816
Mineral interests, net	2,905,002	2,965,557
Property, plant and equipment, net	1,637,552	1,732,131
Deferred income taxes		160,422
Other long-term assets	98,958	94,494
Total assets	5,590,860	5,768,420	6,856,508
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	9,210	18,793
Accounts payable	92,712	114,913
Accrued expenses	133,870	184,875
Accumulated other postretirement benefits obligation	30,036	29,200
Other current liabilities	214,073	206,473
Total current liabilities	479,901	554,254
Long-term debt	2,769,622	2,397,372
Accumulated other postretirement benefits obligation	570,712	633,264
Deferred income taxes	822,867	921,687
Other long-term liabilities	195,064	251,272
Total liabilities	4,838,166	4,757,849
Total stockholders' equity	752,694	1,010,571	2,136,517	595,066
Total liabilities and stockholders' equity	5,590,860	5,768,420
Parent (Issuer)
ASSETS
Cash and cash equivalents	234,150	83,833	99,086	294,413
Receivables, net	113,936	64,106
Intercompany receivables		721,293
Intercompany loans receivable	63,549	118,079
Deferred income taxes	23,957	39,375
Prepaid expenses	2,245	1,869
Other current assets	15,257	17,559
Total current assets	453,094	1,046,114
Property, plant and equipment, net	7,248	8,448
Deferred income taxes	3,049	52,363
Investment in subsidiaries	4,409,683	3,530,094
Other long-term assets	73,564	71,622
Total assets	4,946,638	4,708,641
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	5,604	5,128
Accrued expenses	34,551	27,197
Intercompany payables	87,904
Intercompany loans payable	1,104,282	1,074,879
Accumulated other postretirement benefits obligation	94	131
Other current liabilities	164,364	157,044
Total current liabilities	1,396,799	1,264,379
Long-term debt	2,763,957	2,381,255
Accumulated other postretirement benefits obligation	263	452
Other long-term liabilities	32,925	51,984
Total liabilities	4,193,944	3,698,070
Total stockholders' equity	752,694	1,010,571
Total liabilities and stockholders' equity	4,946,638	4,708,641
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	101	61	79	(1,823)
Receivables, net	90,460	113,182
Intercompany receivables	30,126
Intercompany loans receivable	1,104,282	1,074,879
Inventories	168,434	131,893
Deferred income taxes	12,154	17,687
Prepaid expenses	34,011	45,327
Other current assets	440	1,109
Total current assets	1,440,008	1,384,138
Mineral interests, net	7,294	18,475
Property, plant and equipment, net	764,406	790,900
Deferred income taxes	4,458	112,560
Investment in subsidiaries	86,357
Other long-term assets	10,323	9,375
Total assets	2,312,846	2,315,448
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	1,313	10,196
Accounts payable	64,678	78,260
Accrued expenses	53,582	83,155
Intercompany payables		567,360
Accumulated other postretirement benefits obligation	29,942	29,069
Other current liabilities	27,062	24,389
Total current liabilities	176,577	792,429
Long-term debt		1,784
Accumulated other postretirement benefits obligation	570,449	632,812
Other long-term liabilities	73,420	128,593
Total liabilities	820,446	1,555,618
Total stockholders' equity	1,492,400	759,830
Total liabilities and stockholders' equity	2,312,846	2,315,448
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	26,567	32,707	29,265	820
Receivables, net	77,367	79,679
Intercompany receivables	57,778
Inventories	144,213	174,125
Deferred income taxes	956	1,464
Prepaid expenses	2,766	6,580
Other current assets	2,334	5,260
Total current assets	311,981	299,815
Mineral interests, net	2,897,708	2,947,082
Property, plant and equipment, net	865,898	932,783
Deferred income taxes		(4,501)
Other long-term assets	15,071	13,497
Total assets	4,090,658	4,188,676
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	7,897	8,597
Accounts payable	22,430	31,525
Accrued expenses	45,737	74,523
Intercompany payables		153,933
Intercompany loans payable	63,549	118,079
Other current liabilities	22,647	25,040
Total current liabilities	162,260	411,697
Long-term debt	5,665	14,333
Deferred income taxes	830,374	921,687
Other long-term liabilities	88,719	70,695
Total liabilities	1,087,018	1,418,412
Total stockholders' equity	3,003,640	2,770,264
Total liabilities and stockholders' equity	4,090,658	4,188,676
Eliminations
ASSETS
Intercompany receivables	(87,904)	(721,293)
Intercompany loans receivable	(1,167,831)	(1,192,958)
Total current assets	(1,255,735)	(1,914,251)
Deferred income taxes	(7,507)
Investment in subsidiaries	(4,496,040)	(3,530,094)
Total assets	(5,759,282)	(5,444,345)
LIABILITIES AND STOCKHOLDERS' EQUITY
Intercompany payables	(87,904)	(721,293)
Intercompany loans payable	(1,167,831)	(1,192,958)
Total current liabilities	(1,255,735)	(1,914,251)
Deferred income taxes	(7,507)
Total liabilities	(1,263,242)	(1,914,251)
Total stockholders' equity	(4,496,040)	(3,530,094)
Total liabilities and stockholders' equity	$ (5,759,282)	$ (5,444,345)