Supplemental Guarantor and Non-Guarantor Financial Information (Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
ASSETS
Cash and cash equivalents	$ 267,003	$ 468,532	$ 293,472	$ 260,818
Trade receivables, net	71,670	91,057
Other receivables	128,735	127,037
Intercompany receivables	0	0
Inventories	162,106	201,598
Deferred income taxes	16,247	16,819
Prepaid expenses	52,194	46,190
Other current assets	9,080	9,285
Total current assets	707,035	960,518
Mineral interests, net	446,570	2,836,801
Property, plant and equipment, net	903,685	1,466,297
Investment in subsidiaries	0	0
Other long-term assets	70,422	67,748
Total assets	2,127,712	5,331,364
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Accounts payable	50,752	38,980
Accrued expenses	129,400	125,318
Intercompany payables	0	0
Pension and other postretirement benefits obligation	29,788	29,032
Liability for uncertain tax positions	166,100	164,348
Other current liabilities	53,411	51,604
Current debt	3,025,849	12,327
Total current liabilities	3,455,300	421,609
Long-term debt	0	3,068,878
Deferred income taxes	70,037	730,685
Pension and other postretirement benefits obligation	645,879	641,231
Other long-term liabilities	188,425	187,380
Total liabilities	4,359,641	5,049,783
Total stockholders' equity (deficit)	(2,231,929)	281,581
Total liabilities and stockholders' equity (deficit)	2,127,712	5,331,364
Reportable legal entities | Parent (Issuer)
ASSETS
Cash and cash equivalents	230,340	421,533	255,620	234,150
Trade receivables, net	0	0
Other receivables	125,524	123,659
Intercompany receivables	4,401	0
Inventories	0	0
Deferred income taxes	15,984	15,986
Prepaid expenses	8,118	3,424
Other current assets	164	106
Total current assets	384,531	564,708
Mineral interests, net	0	0
Property, plant and equipment, net	6,618	7,558
Investment in subsidiaries	814,796	3,233,399
Other long-term assets	39,909	43,420
Total assets	1,245,854	3,849,085
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Accounts payable	19,875	2,296
Accrued expenses	66,462	43,088
Intercompany payables	134,300	221,070
Pension and other postretirement benefits obligation	851	95
Liability for uncertain tax positions	161,271	156,486
Other current liabilities	12,001	11,958
Current debt	3,015,052	0
Total current liabilities	3,409,812	434,993
Long-term debt		3,063,121
Deferred income taxes	27,432	23,766
Pension and other postretirement benefits obligation	10,457	10,502
Other long-term liabilities	30,082	35,122
Total liabilities	3,477,783	3,567,504
Total stockholders' equity (deficit)	(2,231,929)	281,581
Total liabilities and stockholders' equity (deficit)	1,245,854	3,849,085
Reportable legal entities | Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	0	1,117	4,666	1,620
Trade receivables, net	70,005	88,959
Other receivables	2,684	2,193
Intercompany receivables	134,300	206,118
Inventories	100,627	110,882
Deferred income taxes	0	0
Prepaid expenses	40,773	40,044
Other current assets	6,673	7,029
Total current assets	355,062	456,342
Mineral interests, net	132,125	135,377
Property, plant and equipment, net	683,146	726,478
Investment in subsidiaries	6,477	6,459
Other long-term assets	13,533	17,857
Total assets	1,190,343	1,342,513
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Accounts payable	27,073	30,697
Accrued expenses	51,039	60,762
Intercompany payables	0	0
Pension and other postretirement benefits obligation	28,937	28,937
Liability for uncertain tax positions	0	0
Other current liabilities	26,238	27,172
Current debt	9,361	7,209
Total current liabilities	142,648	154,777
Long-term debt		5,704
Deferred income taxes	0	0
Pension and other postretirement benefits obligation	635,422	630,729
Other long-term liabilities	100,469	96,599
Total liabilities	878,539	887,809
Total stockholders' equity (deficit)	311,804	454,704
Total liabilities and stockholders' equity (deficit)	1,190,343	1,342,513
Reportable legal entities | Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	36,663	45,882	33,186	25,048
Trade receivables, net	1,665	2,098
Other receivables	527	1,185
Intercompany receivables	0	14,952
Inventories	61,479	90,716
Deferred income taxes	263	833
Prepaid expenses	3,303	2,722
Other current assets	2,243	2,150
Total current assets	106,143	160,538
Mineral interests, net	314,445	2,701,424
Property, plant and equipment, net	213,921	732,261
Investment in subsidiaries	0	0
Other long-term assets	16,980	6,471
Total assets	651,489	3,600,694
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Accounts payable	3,804	5,987
Accrued expenses	11,899	21,468
Intercompany payables	4,401	0
Pension and other postretirement benefits obligation	0	0
Liability for uncertain tax positions	4,829	7,862
Other current liabilities	15,172	12,474
Current debt	1,436	5,118
Total current liabilities	41,541	52,909
Long-term debt		53
Deferred income taxes	42,605	706,919
Pension and other postretirement benefits obligation	0	0
Other long-term liabilities	57,874	55,659
Total liabilities	142,020	815,540
Total stockholders' equity (deficit)	509,469	2,785,154
Total liabilities and stockholders' equity (deficit)	651,489	3,600,694
Eliminations
ASSETS
Cash and cash equivalents	0	0	$ 0	$ 0
Trade receivables, net	0	0
Other receivables	0	0
Intercompany receivables	(138,701)	(221,070)
Inventories	0	0
Deferred income taxes	0	0
Prepaid expenses	0	0
Other current assets	0	0
Total current assets	(138,701)	(221,070)
Mineral interests, net	0	0
Property, plant and equipment, net	0	0
Investment in subsidiaries	(821,273)	(3,239,858)
Other long-term assets	0	0
Total assets	(959,974)	(3,460,928)
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Accounts payable	0	0
Accrued expenses	0	0
Intercompany payables	(138,701)	(221,070)
Pension and other postretirement benefits obligation	0	0
Liability for uncertain tax positions	0	0
Other current liabilities	0	0
Current debt	0	0
Total current liabilities	(138,701)	(221,070)
Long-term debt		0
Deferred income taxes	0	0
Pension and other postretirement benefits obligation	0	0
Other long-term liabilities	0	0
Total liabilities	(138,701)	(221,070)
Total stockholders' equity (deficit)	(821,273)	(3,239,858)
Total liabilities and stockholders' equity (deficit)	$ (959,974)	$ (3,460,928)