Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 293,472	$ 260,818	$ 170,878	$ 116,601
Receivables, net	231,032	281,763
Inventories	274,482	312,647
Deferred income taxes	32,079	37,067
Prepaid expenses	45,361	39,022
Assets held for sale	24,150
Other current assets	12,504	18,031
Total current assets	913,080	949,348
Mineral interests, net	2,887,612	2,905,002
Property, plant and equipment, net	1,547,280	1,637,552
Other long-term assets	113,926	98,958
Total assets	5,461,898	5,590,860
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	15,353	9,210
Accounts payable	60,451	92,712
Accrued expenses	142,831	133,870
Accumulated other postretirement benefits obligation	30,887	30,036
Other current liabilities	225,713	214,073
Total current liabilities	475,235	479,901
Long-term debt	2,880,951	2,769,622
Accumulated other postretirement benefits obligation	574,856	570,712
Deferred income taxes	783,591	822,867
Other long-term liabilities	189,930	195,064
Total liabilities	4,904,563	4,838,166
Total stockholders' equity	557,335	752,694
Total liabilities and stockholders' equity	5,461,898	5,590,860
Reportable legal entities | Parent (Issuer)
ASSETS
Cash and cash equivalents	255,620	234,150	136,634	83,833
Receivables, net	102,008	113,936
Intercompany loans receivable		63,549
Deferred income taxes	26,390	23,957
Prepaid expenses	1,457	2,245
Other current assets	9,666	15,257
Total current assets	395,141	453,094
Property, plant and equipment, net	7,680	7,248
Deferred income taxes	3,046	3,049
Investment in subsidiaries	3,435,398	4,409,683
Other long-term assets	85,925	73,564
Total assets	3,927,190	4,946,638
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	6,968	5,604
Accrued expenses	45,962	34,551
Intercompany payables	239,769	87,904
Intercompany loans payable	352	1,104,282
Accumulated other postretirement benefits obligation	945	94
Other current liabilities	179,634	164,364
Total current liabilities	473,630	1,396,799
Long-term debt	2,869,931	2,763,957
Accumulated other postretirement benefits obligation	(497)	263
Other long-term liabilities	26,791	32,925
Total liabilities	3,369,855	4,193,944
Total stockholders' equity	557,335	752,694
Total liabilities and stockholders' equity	3,927,190	4,946,638
Reportable legal entities | Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	27	101		61
Receivables, net	103,686	90,460
Intercompany receivables	179,412	30,126
Intercompany loans receivable		1,104,282
Inventories	130,447	168,434
Deferred income taxes	4,733	12,154
Prepaid expenses	38,683	34,011
Assets held for sale	24,150
Other current assets	478	440
Total current assets	481,616	1,440,008
Mineral interests, net	6,897	7,294
Property, plant and equipment, net	712,731	764,406
Deferred income taxes	7,990	4,458
Investment in subsidiaries	81,384	86,357
Other long-term assets	18,718	10,323
Total assets	1,309,336	2,312,846
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	7,461	1,313
Accounts payable	42,342	64,678
Accrued expenses	60,237	53,582
Accumulated other postretirement benefits obligation	29,942	29,942
Other current liabilities	21,313	27,062
Total current liabilities	161,295	176,577
Long-term debt	9,355
Accumulated other postretirement benefits obligation	575,353	570,449
Other long-term liabilities	81,079	73,420
Total liabilities	827,082	820,446
Total stockholders' equity	482,254	1,492,400
Total liabilities and stockholders' equity	1,309,336	2,312,846
Reportable legal entities | Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	37,825	26,567	34,244	32,707
Receivables, net	25,338	77,367
Intercompany receivables	60,357	57,778
Intercompany loans receivable	352
Inventories	144,035	144,213
Deferred income taxes	956	956
Prepaid expenses	5,221	2,766
Other current assets	2,360	2,334
Total current assets	276,444	311,981
Mineral interests, net	2,880,715	2,897,708
Property, plant and equipment, net	826,869	865,898
Other long-term assets	9,283	15,071
Total assets	3,993,311	4,090,658
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	7,892	7,897
Accounts payable	11,141	22,430
Accrued expenses	36,632	45,737
Intercompany loans payable		63,549
Other current liabilities	24,766	22,647
Total current liabilities	80,431	162,260
Long-term debt	1,665	5,665
Deferred income taxes	794,627	830,374
Other long-term liabilities	82,060	88,719
Total liabilities	958,783	1,087,018
Total stockholders' equity	3,034,528	3,003,640
Total liabilities and stockholders' equity	3,993,311	4,090,658
Eliminations
ASSETS
Intercompany receivables	(239,769)	(87,904)
Intercompany loans receivable	(352)	(1,167,831)
Total current assets	(240,121)	(1,255,735)
Deferred income taxes	(11,036)	(7,507)
Investment in subsidiaries	(3,516,782)	(4,496,040)
Total assets	(3,767,939)	(5,759,282)
LIABILITIES AND STOCKHOLDERS' EQUITY
Intercompany payables	(239,769)	(87,904)
Intercompany loans payable	(352)	(1,167,831)
Total current liabilities	(240,121)	(1,255,735)
Deferred income taxes	(11,036)	(7,507)
Total liabilities	(251,157)	(1,263,242)
Total stockholders' equity	(3,516,782)	(4,496,040)
Total liabilities and stockholders' equity	$ (3,767,939)	$ (5,759,282)