Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 404,715	$ 260,818	$ 235,791	$ 116,601
Receivables, net	254,874	281,763
Inventories	311,167	312,647
Deferred income taxes	33,864	37,067
Prepaid expenses	29,261	39,022
Other current assets	14,209	18,031
Total current assets	1,048,090	949,348
Mineral interests, net	2,887,136	2,905,002
Property, plant and equipment, net	1,615,419	1,637,552
Other long-term assets	98,020	98,958
Total assets	5,648,665	5,590,860	5,590,860
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	15,379	9,210
Accounts payable	72,213	92,712
Accrued expenses	165,633	133,870
Accumulated other postretirement benefits obligation	30,462	30,036
Other current liabilities	225,851	214,073
Total current liabilities	509,538	479,901
Long-term debt	2,918,066	2,769,622
Accumulated other postretirement benefits obligation	573,324	570,712
Deferred income taxes	790,263	822,867
Other long-term liabilities	187,865	195,064
Total liabilities	4,979,056	4,838,166
Total stockholders' equity	669,609	752,694
Total liabilities and stockholders' equity	5,648,665	5,590,860
Reportable legal entities | Parent (Issuer)
ASSETS
Cash and cash equivalents	379,521	234,150	200,185	83,833
Receivables, net	113,539	113,936
Intercompany loans receivable	13,870	63,549
Deferred income taxes	42,519	23,957
Prepaid expenses	1,924	2,245
Other current assets	11,451	15,257
Total current assets	562,824	453,094
Property, plant and equipment, net	8,127	7,248
Deferred income taxes	3,046	3,049
Investment in subsidiaries	4,540,783	4,409,683
Other long-term assets	72,432	73,564
Total assets	5,187,212	4,946,638
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	7,505	5,604
Accrued expenses	64,566	34,551
Intercompany payables	219,709	87,904
Intercompany loans payable	1,111,632	1,104,282
Accumulated other postretirement benefits obligation	520	94
Other current liabilities	176,892	164,364
Total current liabilities	1,580,824	1,396,799
Long-term debt	2,903,382	2,763,957
Accumulated other postretirement benefits obligation	(138)	263
Other long-term liabilities	33,535	32,925
Total liabilities	4,517,603	4,193,944
Total stockholders' equity	669,609	752,694
Total liabilities and stockholders' equity	5,187,212	4,946,638
Reportable legal entities | Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	48	101	158	61
Receivables, net	102,792	90,460
Intercompany receivables	188,183	30,126
Intercompany loans receivable	1,111,632	1,104,282
Inventories	137,803	168,434
Deferred income taxes		12,154
Prepaid expenses	25,073	34,011
Other current assets	459	440
Total current assets	1,565,990	1,440,008
Mineral interests, net	7,073	7,294
Property, plant and equipment, net	766,133	764,406
Deferred income taxes	17,816	4,458
Investment in subsidiaries	83,770	86,357
Other long-term assets	16,410	10,323
Total assets	2,457,192	2,312,846
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	7,844	1,313
Accounts payable	40,388	64,678
Accrued expenses	65,103	53,582
Accumulated other postretirement benefits obligation	29,942	29,942
Other current liabilities	34,265	27,062
Total current liabilities	177,542	176,577
Long-term debt	11,137
Accumulated other postretirement benefits obligation	573,462	570,449
Other long-term liabilities	74,282	73,420
Total liabilities	836,423	820,446
Total stockholders' equity	1,620,769	1,492,400
Total liabilities and stockholders' equity	2,457,192	2,312,846
Reportable legal entities | Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	25,146	26,567	35,448	32,707
Receivables, net	38,543	77,367
Intercompany receivables	31,526	57,778
Inventories	173,364	144,213
Deferred income taxes	760	956
Prepaid expenses	2,264	2,766
Other current assets	2,299	2,334
Total current assets	273,902	311,981
Mineral interests, net	2,880,063	2,897,708
Property, plant and equipment, net	841,159	865,898
Other long-term assets	9,178	15,071
Total assets	4,004,302	4,090,658
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	7,535	7,897
Accounts payable	24,320	22,430
Accrued expenses	35,964	45,737
Intercompany loans payable	13,870	63,549
Other current liabilities	24,109	22,647
Total current liabilities	105,798	162,260
Long-term debt	3,547	5,665
Deferred income taxes	811,125	830,374
Other long-term liabilities	80,048	88,719
Total liabilities	1,000,518	1,087,018
Total stockholders' equity	3,003,784	3,003,640
Total liabilities and stockholders' equity	4,004,302	4,090,658
Eliminations
ASSETS
Intercompany receivables	(219,709)	(87,904)
Intercompany loans receivable	(1,125,502)	(1,167,831)
Deferred income taxes	(9,415)
Total current assets	(1,354,626)	(1,255,735)
Deferred income taxes	(20,862)	(7,507)
Investment in subsidiaries	(4,624,553)	(4,496,040)
Total assets	(6,000,041)	(5,759,282)
LIABILITIES AND STOCKHOLDERS' EQUITY
Intercompany payables	(219,709)	(87,904)
Intercompany loans payable	(1,125,502)	(1,167,831)
Other current liabilities	(9,415)
Total current liabilities	(1,354,626)	(1,255,735)
Deferred income taxes	(20,862)	(7,507)
Total liabilities	(1,375,488)	(1,263,242)
Total stockholders' equity	(4,624,553)	(4,496,040)
Total liabilities and stockholders' equity	$ (6,000,041)	$ (5,759,282)