SUPPLEMENTAL CONSOLIDATING FINANCIAL INFORMATION - Supplemental Consolidating Balance Sheet Information (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 40,610	$ 61,110	$ 36,274	$ 31,610
Receivables:
Trade	129,652	66,196
Loan and lease receivable	11,682	0
Contractual third-party reclamation receivables	7,940	8,487
Intercompany receivable/payable	0	0
Other	3,182	5,086
Total receivables	152,456	79,769
Inventories	153,154	39,972
Deferred income taxes	6,053	5,355
Restricted investments and bond collateral	0	5,998
Other current assets	12,362	12,835
Total current assets	364,635	205,039
Property, plant and equipment:
Land and mineral rights	439,001	278,188
Plant and equipment	832,337	657,696
Gross property, plant and equipment	1,271,338	935,884
Less accumulated depreciation, depletion and amortization	485,820	445,848
Net property, plant and equipment	785,518	490,036
Loan and lease receivable	85,344	0
Advanced coal royalties	7,134	7,311
Reclamation deposits	75,911	74,921
Restricted investments and bond collateral	104,511	69,235
Contractual third-party reclamation receivables	93,956	88,303
Investment in joint venture	33,273	0
Intangible assets	677	1,520
Investment in subsidiaries	0	0
Other assets	32,762	10,320
Total Assets	1,583,721	946,685	933,603
Current liabilities
Current installments of long-term debt	42,401	44,343
Accounts payable and accrued expenses:
Trade	128,338	57,507
Interest payable	30,750	11,321
Production taxes	42,673	41,905
Workers' compensation	707	717
Postretirement medical benefits	13,955	13,955
SERP	390	390
Deferred revenue	11,561	14,068
Asset retirement obligations	31,131	23,353	24,105
Other current liabilities	11,973	5,469
Total current liabilities	313,879	213,028
Long-term debt, less current installments	790,643	295,494
Workers' compensation, less current portion	6,629	6,744
Excess of black lung benefit obligation over trust assets	9,841	8,675
Postretirement medical benefits, less current portion	271,773	270,374
Pension and SERP obligations, less current portion	22,222	24,176
Deferred revenue, less current portion	40,676	46,567
Asset retirement obligations, less current portion	358,253	256,511	239,046
Intangible liabilities	5,072	5,606
Deferred income taxes	15,514	5,355
Other liabilities	9,864	2,034
Intercompany receivable/payable	0	0
Total liabilities	1,844,366	1,134,564
Shareholders' deficit:
Preferred stock	96	160
Common stock	38,173	36,479
Other paid-in capital	133,140	134,861
Accumulated other comprehensive loss	(53,615)	(63,595)
Accumulated earnings (deficit)	(378,439)	(295,784)
Total equity (deficit)	(260,645)	(187,879)
Total Liabilities and Shareholders' Deficit	1,583,721	946,685
Parent/ Issuer
Current assets:
Cash and cash equivalents	13,514	25,326	15,665	14,836
Receivables:
Trade	0	0
Loan and lease receivable	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	132,824	(16,273)
Other	673	1,616
Total receivables	133,497	(14,657)
Inventories	0	0
Deferred income taxes	5,370	5,355
Restricted investments and bond collateral		0
Other current assets	972	789
Total current assets	153,353	16,813
Property, plant and equipment:
Land and mineral rights	0	0
Plant and equipment	4,022	3,973
Gross property, plant and equipment	4,022	3,973
Less accumulated depreciation, depletion and amortization	2,863	2,707
Net property, plant and equipment	1,159	1,266
Loan and lease receivable	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	63,322	15,134
Contractual third-party reclamation receivables	0	0
Investment in joint venture	0
Intangible assets	0	0
Investment in subsidiaries	480,871	280,843
Other assets	21,825	8,636
Total Assets	720,530	322,692
Current liabilities
Current installments of long-term debt	0	20,392
Accounts payable and accrued expenses:
Trade	5,240	6,840
Interest payable	30,315	11,302
Production taxes	0	0
Workers' compensation	707	717
Postretirement medical benefits	12,042	12,042
SERP	390	390
Deferred revenue	0	0
Asset retirement obligations	0	0
Other current liabilities	593	0
Total current liabilities	49,287	51,683
Long-term debt, less current installments	698,015	224,582
Workers' compensation, less current portion	6,629	6,744
Excess of black lung benefit obligation over trust assets	9,841	8,675
Postretirement medical benefits, less current portion	184,902	185,858
Pension and SERP obligations, less current portion	12,776	13,069
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	0	0
Intangible liabilities	0	0
Deferred income taxes	5,370	5,355
Other liabilities	707	584
Intercompany receivable/payable	13,648	14,021
Total liabilities	981,175	510,571
Shareholders' deficit:
Preferred stock	96	160
Common stock	38,173	36,479
Other paid-in capital	133,140	134,861
Accumulated other comprehensive loss	(53,615)	(63,595)
Accumulated earnings (deficit)	(378,439)	(295,784)
Total equity (deficit)	(260,645)	(187,879)
Total Liabilities and Shareholders' Deficit	720,530	322,692
Co-Issuer
Current assets:
Cash and cash equivalents	415	3,341	476	4,545
Receivables:
Trade	12,940	12,934
Loan and lease receivable	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	32,639	0
Other	210	210
Total receivables	45,789	13,144
Inventories	5,319	6,161
Deferred income taxes	0	0
Restricted investments and bond collateral		5,998
Other current assets	335	143
Total current assets	51,858	28,787
Property, plant and equipment:
Land and mineral rights	1,395	1,395
Plant and equipment	221,350	220,872
Gross property, plant and equipment	222,745	222,267
Less accumulated depreciation, depletion and amortization	76,662	71,653
Net property, plant and equipment	146,083	150,614
Loan and lease receivable	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	0	0
Contractual third-party reclamation receivables	0	0
Investment in joint venture	0
Intangible assets	460	1,283
Investment in subsidiaries	0	0
Other assets	0	0
Total Assets	198,401	180,684
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade	7,996	10,119
Interest payable	0	0
Production taxes	1,081	3
Workers' compensation	0	0
Postretirement medical benefits	0	0
SERP	0	0
Deferred revenue	9,581	9,024
Asset retirement obligations	0	0
Other current liabilities	5,279	5,053
Total current liabilities	23,937	24,199
Long-term debt, less current installments	0	0
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	0	0
Pension and SERP obligations, less current portion	97	99
Deferred revenue, less current portion	35,949	41,297
Asset retirement obligations, less current portion	927	892
Intangible liabilities	5,072	5,606
Deferred income taxes	0	0
Other liabilities	5,790	0
Intercompany receivable/payable	0	0
Total liabilities	71,772	72,093
Shareholders' deficit:
Preferred stock	0	0
Common stock	5	5
Other paid-in capital	52,848	52,835
Accumulated other comprehensive loss	(160)	(164)
Accumulated earnings (deficit)	73,936	55,915
Total equity (deficit)	126,629	108,591
Total Liabilities and Shareholders' Deficit	198,401	180,684
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	24,440	27,451	16,416	10,237
Receivables:
Trade	116,712	46,985
Loan and lease receivable	11,682
Contractual third-party reclamation receivables	7,940	8,487
Intercompany receivable/payable	(146,187)	2,761
Other	2,294	3,256
Total receivables	(7,559)	61,489
Inventories	147,834	33,811
Deferred income taxes	683	870
Restricted investments and bond collateral		0
Other current assets	11,055	11,069
Total current assets	176,453	134,690
Property, plant and equipment:
Land and mineral rights	437,606	276,793
Plant and equipment	606,965	432,851
Gross property, plant and equipment	1,044,571	709,644
Less accumulated depreciation, depletion and amortization	406,295	371,488
Net property, plant and equipment	638,276	338,156
Loan and lease receivable	85,344
Advanced coal royalties	7,134	7,311
Reclamation deposits	75,911	74,921
Restricted investments and bond collateral	41,189	54,101
Contractual third-party reclamation receivables	93,956	88,303
Investment in joint venture	33,273
Intangible assets	217	237
Investment in subsidiaries	0	0
Other assets	10,937	1,683
Total Assets	1,162,690	699,402
Current liabilities
Current installments of long-term debt	42,401	23,951
Accounts payable and accrued expenses:
Trade	114,423	38,061
Interest payable	474	40
Production taxes	38,936	36,522
Workers' compensation	0	0
Postretirement medical benefits	773	774
SERP	0	0
Deferred revenue	1,980	5,044
Asset retirement obligations	31,131	23,353
Other current liabilities	6,102	417
Total current liabilities	236,220	128,162
Long-term debt, less current installments	96,128	72,912
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	68,766	66,439
Pension and SERP obligations, less current portion	9,110	10,765
Deferred revenue, less current portion	4,728	5,270
Asset retirement obligations, less current portion	357,326	255,619
Intangible liabilities	0	0
Deferred income taxes	10,144	0
Other liabilities	3,167	6,687
Intercompany receivable/payable	(13,849)	(7,851)
Total liabilities	771,740	538,003
Shareholders' deficit:
Preferred stock	0	0
Common stock	110	110
Other paid-in capital	441,269	157,984
Accumulated other comprehensive loss	16,075	6,425
Accumulated earnings (deficit)	(66,504)	(3,120)
Total equity (deficit)	390,950	161,399
Total Liabilities and Shareholders' Deficit	1,162,690	699,402
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	2,241	4,992	3,717	1,992
Receivables:
Trade	0	6,277
Loan and lease receivable	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	(19,275)	(19,353)
Other	45	28
Total receivables	(19,230)	(13,048)
Inventories	1	0
Deferred income taxes	0	0
Restricted investments and bond collateral		0
Other current assets	0	834
Total current assets	(16,988)	(7,222)
Property, plant and equipment:
Land and mineral rights	0	0
Plant and equipment	0	0
Gross property, plant and equipment	0	0
Less accumulated depreciation, depletion and amortization	0	0
Net property, plant and equipment	0	0
Loan and lease receivable	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	0	0
Contractual third-party reclamation receivables	0	0
Investment in joint venture	0
Intangible assets	0	0
Investment in subsidiaries	3,770	3,770
Other assets	3,500	2,000
Total Assets	(9,718)	(1,452)
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade	684	2,487
Interest payable	0	0
Production taxes	2,657	5,380
Workers' compensation	0	0
Postretirement medical benefits	1,139	1,139
SERP	0	0
Deferred revenue	0	0
Asset retirement obligations	0	0
Other current liabilities	0	0
Total current liabilities	4,480	9,006
Long-term debt, less current installments	0	0
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	18,105	18,077
Pension and SERP obligations, less current portion	239	243
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	0	0
Intangible liabilities	0	0
Deferred income taxes	0	0
Other liabilities	200	983
Intercompany receivable/payable	14,654	14,654
Total liabilities	37,678	42,963
Shareholders' deficit:
Preferred stock	0	0
Common stock	132	132
Other paid-in capital	(124)	(124)
Accumulated other comprehensive loss	(3,067)	(3,086)
Accumulated earnings (deficit)	(44,337)	(41,337)
Total equity (deficit)	(47,396)	(44,415)
Total Liabilities and Shareholders' Deficit	(9,718)	(1,452)
Consolidating Adjustments
Current assets:
Cash and cash equivalents	0	0	0	0
Receivables:
Trade	0	0
Loan and lease receivable	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	(1)	32,865
Other	(40)	(24)
Total receivables	(41)	32,841
Inventories	0	0
Deferred income taxes	0	(870)
Restricted investments and bond collateral		0
Other current assets	0	0
Total current assets	(41)	31,971
Property, plant and equipment:
Land and mineral rights	0	0
Plant and equipment	0	0
Gross property, plant and equipment	0	0
Less accumulated depreciation, depletion and amortization	0	0
Net property, plant and equipment	0	0
Loan and lease receivable	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	0	0
Contractual third-party reclamation receivables	0	0
Investment in joint venture	0
Intangible assets	0	0
Investment in subsidiaries	(484,641)	(284,613)
Other assets	(3,500)	(1,999)
Total Assets	(488,182)	(254,641)
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade	(5)	0
Interest payable	(39)	(21)
Production taxes	(1)	0
Workers' compensation	0	0
Postretirement medical benefits	1	0
SERP	0	0
Deferred revenue	0	0
Asset retirement obligations	0	0
Other current liabilities	(1)	(1)
Total current liabilities	(45)	(22)
Long-term debt, less current installments	(3,500)	(2,000)
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	0	0
Pension and SERP obligations, less current portion	0	0
Deferred revenue, less current portion	(1)	0
Asset retirement obligations, less current portion	0	0
Intangible liabilities	0	0
Deferred income taxes	0	0
Other liabilities	0	(6,220)
Intercompany receivable/payable	(14,453)	(20,824)
Total liabilities	(17,999)	(29,066)
Shareholders' deficit:
Preferred stock	0	0
Common stock	(247)	(247)
Other paid-in capital	(493,993)	(210,695)
Accumulated other comprehensive loss	(12,848)	(3,175)
Accumulated earnings (deficit)	36,905	(11,458)
Total equity (deficit)	(470,183)	(225,575)
Total Liabilities and Shareholders' Deficit	$ (488,182)	$ (254,641)