SUPPLEMENTAL CONSOLIDATING FINANCIAL INFORMATION - Supplemental Consolidating Balance Sheet Information (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 123,690	$ 61,110	$ 45,487	$ 31,610
Receivables:
Trade	138,779	66,196
Loan and lease receivables	10,332	0
Contractual third-party reclamation receivables	7,666	8,487
Intercompany receivable/payable	0	0
Other	2,437	5,086
Total receivables	159,214	79,769
Inventories	114,225	39,972
Deferred income taxes	2,226	5,355
Restricted investments and bond collateral	0	5,998
Other current assets	14,602	12,835
Total current assets	413,957	205,039
Property, plant and equipment:
Land and mineral rights	444,701	278,188
Plant and equipment	801,967	657,696
Gross property, plant and equipment	1,246,668	935,884
Less accumulated depreciation, depletion and amortization	503,929	445,848
Net property, plant and equipment	742,739	490,036
Loan and lease receivables	76,080	0
Advanced coal royalties	7,533	7,311
Reclamation deposits	77,131	74,921
Restricted investments and bond collateral	108,734	69,235
Contractual third-party reclamation receivables	90,248	88,303
Investment in joint venture	30,766	0
Intangible assets	335	1,520
Investment in subsidiaries	0	0
Intercompany receivable/payable	0
Other assets	30,985	10,320
Total Assets	1,578,508	946,685	939,839
Current liabilities
Current installments of long-term debt	40,931	44,343
Accounts payable and accrued expenses:
Trade and other accrued liabilities	140,946	57,507
Interest payable	12,510	11,321
Production taxes	50,427	41,905
Workers' compensation	702	717
Postretirement medical benefits	13,955	13,955
SERP	390	390
Deferred revenue	13,554	14,068
Asset retirement obligations	29,529	23,353	25,038
Intercompany receivable/ payable	0
Other current liabilities	9,775	5,469
Total current liabilities	312,719	213,028
Long-term debt, less current installments	777,999	295,494
Workers' compensation, less current portion	6,580	6,744
Excess of black lung benefit obligation over trust assets	10,768	8,675
Postretirement medical benefits, less current portion	272,523	270,374
Pension and SERP obligations, less current portion	20,772	24,176
Deferred revenue, less current portion	37,984	46,567
Asset retirement obligations, less current portion	355,278	256,511	238,163
Intangible liabilities	4,805	5,606
Deferred income taxes	12,325	5,355
Other liabilities	31,086	2,034
Intercompany receivable/payable	0	0
Total liabilities	1,842,839	1,134,564
Shareholders' deficit:
Preferred stock	92	160
Common stock	42,649	36,479
Other paid-in capital	188,214	134,861
Advances to Parent	0
Accumulated other comprehensive loss	(67,520)	(63,595)
Accumulated earnings (deficit)	(427,766)	(295,784)
Total shareholders’ deficit	(264,331)	(187,879)
Total Liabilities and Shareholders' Deficit	1,578,508	946,685
Parent/ Issuer
Current assets:
Cash and cash equivalents	95,245	25,326	8,647	14,836
Receivables:
Trade	0	0
Loan and lease receivables	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	195	(16,273)
Other	1,856	1,616
Total receivables	2,051	(14,657)
Inventories	0	0
Deferred income taxes	5,455	5,355
Restricted investments and bond collateral		0
Other current assets	926	789
Total current assets	103,677	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	2,933	2,707
Net property, plant and equipment	1,089	1,266
Loan and lease receivables	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	63,602	15,134
Contractual third-party reclamation receivables	0	0
Investment in joint venture	0
Intangible assets	0	0
Investment in subsidiaries	337,967	280,843
Intercompany receivable/payable	171,304
Other assets	21,277	8,636
Total Assets	698,916	322,692
Current liabilities
Current installments of long-term debt	0	20,392
Accounts payable and accrued expenses:
Trade and other accrued liabilities	6,167	6,840
Interest payable	12,166	11,302
Production taxes	0	0
Workers' compensation	702	717
Postretirement medical benefits	12,042	12,042
SERP	390	390
Deferred revenue	0	0
Asset retirement obligations	0	0
Intercompany receivable/ payable	0
Other current liabilities	1,080	0
Total current liabilities	32,547	51,683
Long-term debt, less current installments	696,833	224,582
Workers' compensation, less current portion	6,580	6,744
Excess of black lung benefit obligation over trust assets	10,768	8,675
Postretirement medical benefits, less current portion	184,323	185,858
Pension and SERP obligations, less current portion	12,634	13,069
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	0	0
Intangible liabilities	0	0
Deferred income taxes	5,455	5,355
Other liabilities	707	584
Intercompany receivable/payable	13,400	14,021
Total liabilities	963,247	510,571
Shareholders' deficit:
Preferred stock	92	160
Common stock	42,649	36,479
Other paid-in capital	188,214	134,861
Advances to Parent	0
Accumulated other comprehensive loss	(67,520)	(63,595)
Accumulated earnings (deficit)	(427,766)	(295,784)
Total shareholders’ deficit	(264,331)	(187,879)
Total Liabilities and Shareholders' Deficit	698,916	322,692
Co-Issuer
Current assets:
Cash and cash equivalents	0	3,341	907	4,545
Receivables:
Trade	12,173	12,934
Loan and lease receivables	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	0	0
Other	0	210
Total receivables	12,173	13,144
Inventories	3,425	6,161
Deferred income taxes	0	0
Restricted investments and bond collateral		5,998
Other current assets	640	143
Total current assets	16,238	28,787
Property, plant and equipment:
Land and mineral rights	1,395	1,395
Plant and equipment	221,530	220,872
Gross property, plant and equipment	222,925	222,267
Less accumulated depreciation, depletion and amortization	79,165	71,653
Net property, plant and equipment	143,760	150,614
Loan and lease receivables	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	3,400	0
Contractual third-party reclamation receivables	0	0
Investment in joint venture	0
Intangible assets	129	1,283
Investment in subsidiaries	0	0
Intercompany receivable/payable	0
Other assets	0	0
Total Assets	163,527	180,684
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade and other accrued liabilities	7,964	10,119
Interest payable	0	0
Production taxes	2	3
Workers' compensation	0	0
Postretirement medical benefits	0	0
SERP	0	0
Deferred revenue	9,581	9,024
Asset retirement obligations	0	0
Intercompany receivable/ payable	0
Other current liabilities	5,099	5,053
Total current liabilities	22,646	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	96	99
Deferred revenue, less current portion	33,573	41,297
Asset retirement obligations, less current portion	944	892
Intangible liabilities	4,805	5,606
Deferred income taxes	0	0
Other liabilities	25,415	0
Intercompany receivable/payable	0	0
Total liabilities	87,479	72,093
Shareholders' deficit:
Preferred stock	0	0
Common stock	5	5
Other paid-in capital	52,855	52,835
Advances to Parent	8,530
Accumulated other comprehensive loss	(158)	(164)
Accumulated earnings (deficit)	14,816	55,915
Total shareholders’ deficit	76,048	108,591
Total Liabilities and Shareholders' Deficit	163,527	180,684
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	27,974	27,451	33,058	10,237
Receivables:
Trade	126,606	46,985
Loan and lease receivables	10,332
Contractual third-party reclamation receivables	7,666	8,487
Intercompany receivable/payable	376	2,761
Other	570	3,256
Total receivables	145,550	61,489
Inventories	110,800	33,811
Deferred income taxes	865	870
Restricted investments and bond collateral		0
Other current assets	11,423	11,069
Total current assets	296,612	134,690
Property, plant and equipment:
Land and mineral rights	443,306	276,793
Plant and equipment	576,415	432,851
Gross property, plant and equipment	1,019,721	709,644
Less accumulated depreciation, depletion and amortization	421,831	371,488
Net property, plant and equipment	597,890	338,156
Loan and lease receivables	76,080
Advanced coal royalties	7,533	7,311
Reclamation deposits	77,131	74,921
Restricted investments and bond collateral	41,732	54,101
Contractual third-party reclamation receivables	90,248	88,303
Investment in joint venture	30,766
Intangible assets	206	237
Investment in subsidiaries	10,333	0
Intercompany receivable/payable	(171,304)
Other assets	9,708	1,683
Total Assets	1,066,935	699,402
Current liabilities
Current installments of long-term debt	40,931	23,951
Accounts payable and accrued expenses:
Trade and other accrued liabilities	126,754	38,061
Interest payable	392	40
Production taxes	48,440	36,522
Workers' compensation	0	0
Postretirement medical benefits	773	774
SERP	0	0
Deferred revenue	3,973	5,044
Asset retirement obligations	29,529	23,353
Intercompany receivable/ payable	13,664
Other current liabilities	5,596	417
Total current liabilities	270,052	128,162
Long-term debt, less current installments	84,666	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	70,113	66,439
Pension and SERP obligations, less current portion	7,805	10,765
Deferred revenue, less current portion	4,411	5,270
Asset retirement obligations, less current portion	354,335	255,619
Intangible liabilities	0	0
Deferred income taxes	6,870	0
Other liabilities	3,167	6,687
Intercompany receivable/payable	(12,052)	(7,851)
Total liabilities	789,367	538,003
Shareholders' deficit:
Preferred stock	0	0
Common stock	10,379	110
Other paid-in capital	373,650	157,984
Advances to Parent	(24,334)
Accumulated other comprehensive loss	3,068	6,425
Accumulated earnings (deficit)	(85,195)	(3,120)
Total shareholders’ deficit	277,568	161,399
Total Liabilities and Shareholders' Deficit	1,066,935	699,402
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	2,469	4,992	2,875	1,992
Receivables:
Trade	0	6,277
Loan and lease receivables	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	(19,513)	(19,353)
Other	59	28
Total receivables	(19,454)	(13,048)
Inventories	0	0
Deferred income taxes	0	0
Restricted investments and bond collateral		0
Other current assets	1,613	834
Total current assets	(15,372)	(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 receivables	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
Intercompany receivable/payable	0
Other assets	3,500	2,000
Total Assets	(8,102)	(1,452)
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade and other accrued liabilities	63	2,487
Interest payable	(1)	0
Production taxes	1,985	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
Intercompany receivable/ payable	0
Other current liabilities	0	0
Total current liabilities	3,186	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	18,087	18,077
Pension and SERP obligations, less current portion	237	243
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	(1)	0
Intangible liabilities	0	0
Deferred income taxes	0	0
Other liabilities	1,797	983
Intercompany receivable/payable	14,654	14,654
Total liabilities	37,960	42,963
Shareholders' deficit:
Preferred stock	0	0
Common stock	132	132
Other paid-in capital	(124)	(124)
Advances to Parent	0
Accumulated other comprehensive loss	(3,058)	(3,086)
Accumulated earnings (deficit)	(43,012)	(41,337)
Total shareholders’ deficit	(46,062)	(44,415)
Total Liabilities and Shareholders' Deficit	(8,102)	(1,452)
Consolidating Adjustments
Current assets:
Cash and cash equivalents	(1,998)	0	0	0
Receivables:
Trade	0	0
Loan and lease receivables	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	18,942	32,865
Other	(48)	(24)
Total receivables	18,894	32,841
Inventories	0	0
Deferred income taxes	(4,094)	(870)
Restricted investments and bond collateral		0
Other current assets	0	0
Total current assets	12,802	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 receivables	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	(352,070)	(284,613)
Intercompany receivable/payable	0
Other assets	(3,500)	(1,999)
Total Assets	(342,768)	(254,641)
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade and other accrued liabilities	(2)	0
Interest payable	(47)	(21)
Production taxes	0	0
Workers' compensation	0	0
Postretirement medical benefits	1	0
SERP	0	0
Deferred revenue	0	0
Asset retirement obligations	0	0
Intercompany receivable/ payable	(13,664)
Other current liabilities	(2,000)	(1)
Total current liabilities	(15,712)	(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	0	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	(16,002)	(20,824)
Total liabilities	(35,214)	(29,066)
Shareholders' deficit:
Preferred stock	0	0
Common stock	(10,516)	(247)
Other paid-in capital	(426,381)	(210,695)
Advances to Parent	15,804
Accumulated other comprehensive loss	148	(3,175)
Accumulated earnings (deficit)	113,391	(11,458)
Total shareholders’ deficit	(307,554)	(225,575)
Total Liabilities and Shareholders' Deficit	$ (342,768)	$ (254,641)