Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015	Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 60,082	$ 28,914		$ 35,876		$ 17,754		$ 22,936	$ 29,336		$ 35,876		$ 53,393		$ 14,258	$ 61,110
Receivables:
Trade	140,731	140,063		142,587		150,068		134,141	146,522		136,720		154,167
Loan and lease receivables	5,867	5,394		5,851		5,968		6,157	6,304		9,258		9,609
Other	13,261	10,010		9,567		9,615		11,627	15,114		14,532		16,333
Total receivables	159,859	155,467		158,005		165,651		151,925	167,940		160,510		180,109
Inventories	125,515	129,907		130,215		143,765		122,156	124,451		138,822		134,036
Other current assets	32,258	24,711		19,823		19,951		16,103	20,243		19,521		21,024
Total current assets	377,714	338,999		343,919		347,121		313,120	341,970		354,729		388,562
Property, plant and equipment:
Land and mineral rights	744,253	743,456		741,826		740,940		576,313	610,341		619,762		611,354
Plant and equipment	873,685	879,718		875,122		869,901		790,677	1,012,900		1,006,901		989,914
Gross property, plant and equipment	1,617,938	1,623,174		1,616,948		1,610,841		1,366,990	1,623,241		1,626,663		1,601,268
Less accumulated depreciation, depletion and amortization	782,417	720,210		683,475		655,206		620,148	689,909		657,029		621,434
Net property, plant and equipment	835,521	902,964		933,473		955,635		746,842	933,332		969,634		979,834
Loan and lease receivables	44,474	49,389		50,161		51,823		49,313	51,099		58,627		65,417
Advanced coal royalties	18,722	17,470		17,206		16,367		19,781	17,958		18,725		19,637
Reclamation deposits	74,362	74,043		73,434		77,807		77,364	77,425		76,952		76,715
Restricted investments and bond collateral	144,913	144,454		144,061		143,345		140,807	137,672		128,167		129,813
Investment in joint venture	26,951	27,815		28,045		29,014		27,374	28,664		32,465		32,395
Intangible assets, net of accumulated amortization of $4.6 million at December 31, 2016 and $15.9 million at December 31, 2015	28,199	27,492	[1]	28,050	[1]	28,574	[1]	29,190	29,720	[2]	30,254	[2]	30,784	[2]
Other assets	34,053	23,428		17,678		13,262		12,188	9,638		10,626		12,049
Total Assets	1,584,909	1,606,054		1,636,027		1,662,948		1,415,979	1,627,478		1,680,179		1,735,206		1,740,389
Current liabilities:
Current installments of long-term debt	86,272	90,736		87,754		77,375		38,852	38,879		42,566		42,554
Revolving lines of credit	0	0		3,000		0		1,970	0		2,500		0
Accounts payable and accrued expenses:
Trade and other accrued liabilities	142,233	122,244		135,402		136,989		109,985	129,766		127,313		141,929
Interest payable	22,458	13,611		20,386		11,749		15,527	7,869		16,911		9,180
Production taxes	44,995	55,589		46,797		54,215		46,895	53,437		46,756		52,174
Postretirement medical benefits	14,892	13,855		13,855		13,855		13,855	13,263		13,263		13,263
Deferred revenue	15,253	23,203		19,834		20,303		10,715	13,170		13,176		17,372
Asset retirement obligations	32,207	51,088		50,944		49,445		40,571	47,462		49,860		48,024
Other current liabilities	20,964	24,651		20,451		27,345		31,056	26,551		27,838		28,072
Total current liabilities	379,274	395,345		398,791		391,644		309,426	330,765		340,551		352,936
Long-term debt, less current installments	1,022,794	1,035,013		1,047,244		1,051,674		979,357	987,262		963,488		977,556
Workers’ compensation, less current portion	4,499	4,908		4,992		5,034		5,068	6,081		6,148		6,223
Excess of black lung benefit obligation over trust assets	17,594	17,865		17,594		17,423		17,220	11,919		11,638		11,916
Postretirement medical benefits, less current portion	308,709	286,952		286,739		288,437		285,518	293,268		293,340		293,253
Pension and SERP obligations, less current portion	43,982	42,790		43,702		44,221		44,808	44,256		44,925		48,226
Deferred revenue, less current portion	16,251	18,740		22,441		21,986		24,613	27,425		30,097		32,914
Asset retirement obligations, less current portion	451,834	427,029		424,724		423,996		379,192	402,145		401,403		399,378
Intangible liabilities, net of accumulated amortization of $10.8 million at December 31, 2016 and $9.8 million at December 31, 2015	2,402	2,669	[3]	2,936	[3]	3,203	[3]	3,470	3,737	[4]	4,004	[4]	4,271	[4]
Other liabilities	27,687	33,470		27,648		29,859		30,208	37,014		32,268		28,046
Total liabilities	2,275,026	2,264,781		2,276,811		2,277,477		2,078,880	2,176,798		2,158,322		2,177,406
Shareholders' deficit:
Common stock of $0.01 par value as of December 31, 2015 and $2.50 par value as of December 31, 2014. Authorized 30,000,000 shares; Issued and outstanding 18,162,148 shares at December 31, 2015 and 17,102,777 shares at December 31, 2014, respectively	186	186	[5]	186	[5]	184	[5]	182	180	[6]	180	[6]	44,421	[6]
Other paid-in capital	248,143	246,450		245,050		243,297		240,721	238,705		228,362		184,475
Accumulated other comprehensive loss	(179,072)	(153,951)		(153,484)		(155,122)		(174,270)	(169,629)		(149,608)		(153,708)
Accumulated deficit	(757,367)	(749,817)		(731,448)		(702,858)		(730,266)	(620,090)		(568,672)		(530,503)
Total shareholders’ deficit	(688,110)	(657,132)		(639,696)		(614,499)		(663,633)	(550,834)		(489,738)		(455,315)
Noncontrolling interests in consolidated subsidiaries	(2,007)	(1,595)		(1,088)		(30)		732	1,514		11,595		13,115
Total deficit	(690,117)	(658,727)		(640,784)		(614,529)		(662,901)	(549,320)		(478,143)		(442,200)		$ (400,875)	$ (236,119)
Total Liabilities and Deficit	$ 1,584,909	$ 1,606,054		$ 1,636,027		$ 1,662,948		$ 1,415,979	$ 1,627,478		$ 1,680,179		$ 1,735,206

[1] Intangible assets, net of accumulated amortization of $4.0 million, $3.4 million and $2.9 million at September 30, 2016, June 30, 2016 and March 31, 2016, respectively.

[2] Intangible assets, net of accumulated amortization of $16.9 million, $16.3 million and $15.8 million at September 30, 2015, June 30, 2015 and March 31, 2015, respectively.

[3] Intangible liabilities, net of accumulated amortization of $10.6 million, $10.3 million and $10.0 million at September 30, 2016, June 30, 2016 and March 31, 2016, respectively.

[4] Intangible liabilities, net of accumulated amortization of $14.3 million, $14.0 million and $13.8 million at September 30, 2016, June 30, 2016 and March 31, 2016, respectively.

[5] Common stock of $0.01 par value as of September 30, 2016, June 30, 2016 and March 31, 2016. Authorized 30,000,000 shares; Issued and outstanding 18,570,642 shares at September 30, 2016, 18,569,845 at June 30, 2016 and 18,402,961 at March 31, 2016.

[6] Common stock of $0.01 par value as of September 30, 2015, June 30, 2015 and $2.50 par value at March 31, 2015. Authorized 30,000,000 shares; Issued and outstanding 18,021,061 shares at September 30, 2015, 17,952,320 at June 30, 2015 and 17,769,746 at March 31, 2015