Supplemental Guarantor/Non-Guarantor Financial Information (Details 1) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	$ 248,884	$ 217,073	$ 673,813	$ 572,510
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	204,301	164,287	567,610	469,938
Depreciation and depletion	18,789	23,460	56,683	23,052
Accretion	1,865	1,065	5,532	5,641
(Gain) loss on derivative financial instruments	(838)	1,068	3,102	(5,257)
Selling, general and administrative expenses	12,798	11,161	33,077	38,187
Impairments	0	312	0	4,499
Debt restructuring costs	0	4,499	23	4,499
Other operating costs	121	360	406	814
Total costs and expenses	237,036	206,212	666,433	541,373
Operating income (loss)	11,848	10,861	7,380	31,137
Other income (expense)
Interest income	147	46	304	116
Interest expense	(9,573)	(13,032)	(32,351)	(35,371)
Other, net	(98)	(165)	(546)	(760)
Total other income (expense)	(9,524)	(13,151)	(32,593)	(36,015)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	2,324	(2,290)	(25,213)	(4,878)
Income tax benefit (expense)	115	647	(36)	3,226
Income (loss) from unconsolidated affiliates, net of tax	138	59	771	(1,018)
Net income (loss)	2,577	(1,584)	(24,478)	(2,670)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(1,821)	(1,872)	(5,462)	(3,381)
Postretirement medical plan change		0	0	42,851
Income tax on postretirement medical and pension changes	0	(831)	0	(2,776)
Other comprehensive income (loss)	(1,821)	(2,703)	(5,462)	36,694
Total comprehensive income (loss)	756	(4,287)	(29,940)	34,024
Eliminations
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	(2,238)	(2,438)	(5,887)	(6,581)
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)			0	0
Depreciation and depletion			0	0
Accretion			0	0
(Gain) loss on derivative financial instruments			0	0
Selling, general and administrative expenses	(2,238)	(2,438)	(5,887)	(6,581)
Impairments			0	0
Other operating costs			0	0
Total costs and expenses	(2,238)	(2,438)	(5,887)	(6,581)
Operating income (loss)	0		0	0
Other income (expense)
Interest income			0	0
Interest expense			0	0
Other, net			0	0
Total other income (expense)			0	0
Income (loss) before income tax provision and earnings from unconsolidated affiliates	0		0	0
Income tax benefit (expense)			0	0
Income (loss) from unconsolidated affiliates, net of tax			0	0
Income (loss) from consolidated affiliates, net of tax	(24,735)	(23,353)	(9,822)	(67,760)
Net income (loss)	(24,735)	(23,353)	(9,822)	(67,760)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	3,642	3,744	10,924	6,762
Postretirement medical plan change		0	0	(85,702)
Income tax on postretirement medical and pension changes		1,662	0	5,552
Other comprehensive income (loss)	3,642	5,406	10,924	(73,388)
Total comprehensive income (loss)	(21,093)	(17,947)	1,102	(141,148)
Parent Guarantor (CPE Inc.) | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	2,238	2,438	5,887	6,581
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	0	0	0	0
Depreciation and depletion	0	0	0	0
Accretion	0	0	0	0
(Gain) loss on derivative financial instruments	0	0	0	0
Selling, general and administrative expenses	0	0	0	0
Impairments	0	0	0	0
Other operating costs	0	0	0	0
Total costs and expenses	0	0	0	0
Operating income (loss)	2,238	2,438	5,887	6,581
Other income (expense)
Interest income	0	26	1	27
Interest expense	0	(22)	(48)	(245)
Other, net	0	0	0	0
Total other income (expense)	0	4	(47)	(218)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	2,238	2,442	5,840	6,363
Income tax benefit (expense)	115	(185)	(36)	451
Income (loss) from unconsolidated affiliates, net of tax	0	0	0	0
Income (loss) from consolidated affiliates, net of tax	224	(3,841)	(30,282)	(9,484)
Net income (loss)	2,577	(1,584)	(24,478)	(2,670)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(1,821)	(1,872)	(5,462)	(3,381)
Postretirement medical plan change		0	0	42,851
Income tax on postretirement medical and pension changes	0	(831)	0	(2,776)
Other comprehensive income (loss)	(1,821)	(2,703)	(5,462)	36,694
Total comprehensive income (loss)	756	(4,287)	(29,940)	34,024
Issuers | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	0	0	0	0
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	2	22	9	120
Depreciation and depletion	245	285	626	948
Accretion	0	0	0	0
(Gain) loss on derivative financial instruments	0	0	0	0
Selling, general and administrative expenses	15,036	13,599	38,964	44,768
Impairments	0	0	0	2,048
Debt restructuring costs		4,499	23	4,499
Other operating costs	0	0	0	0
Total costs and expenses	15,283	18,405	39,622	52,383
Operating income (loss)	(15,283)	(18,405)	(39,622)	(52,383)
Other income (expense)
Interest income	147	20	303	89
Interest expense	(9,243)	(12,718)	(31,309)	(34,517)
Other, net	(54)	(29)	(183)	(112)
Total other income (expense)	(9,150)	(12,727)	(31,189)	(34,540)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	(24,433)	(31,132)	(70,811)	(86,923)
Income tax benefit (expense)	0	0	0	0
Income (loss) from unconsolidated affiliates, net of tax	5	6	15	16
Income (loss) from consolidated affiliates, net of tax	24,652	27,285	40,514	77,423
Net income (loss)	224	(3,841)	(30,282)	(9,484)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(1,821)	(1,872)	(5,462)	(3,381)
Postretirement medical plan change	0	0	0	42,851
Income tax on postretirement medical and pension changes	0	(831)	0	(2,776)
Other comprehensive income (loss)	(1,821)	(2,703)	(5,462)	36,694
Total comprehensive income (loss)	(1,597)	(6,544)	(35,744)	27,210
Guarantor Subsidiaries | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	248,884	217,073	673,813	572,510
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	204,299	164,265	567,598	469,818
Depreciation and depletion	18,544	23,175	56,057	22,104
Accretion	1,865	1,065	5,532	5,641
(Gain) loss on derivative financial instruments	(838)	1,068	3,102	(5,257)
Selling, general and administrative expenses	0	0	0	0
Impairments	0	312	0	2,451
Other operating costs	121	360	406	814
Total costs and expenses	223,991	190,245	632,695	495,571
Operating income (loss)	24,893	26,828	41,118	76,939
Other income (expense)
Interest income	0	0	0	0
Interest expense	(135)	(173)	(404)	(318)
Other, net	(98)	(165)	(546)	(760)
Total other income (expense)	(233)	(338)	(950)	(1,078)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	24,660	26,490	40,168	75,861
Income tax benefit (expense)	0	832	0	2,775
Income (loss) from unconsolidated affiliates, net of tax	133	53	756	(1,034)
Income (loss) from consolidated affiliates, net of tax	(141)	(91)	(410)	(179)
Net income (loss)	24,652	27,284	40,514	77,423
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(1,821)	(1,872)	(5,462)	(3,381)
Postretirement medical plan change	0	0	0	42,851
Income tax on postretirement medical and pension changes	0	(831)	0	(2,776)
Other comprehensive income (loss)	(1,821)	(2,703)	(5,462)	36,694
Total comprehensive income (loss)	22,831	24,581	35,052	114,117
Non-Guarantor Subsidiaries | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	0	0	0	0
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	0	0	3	0
Depreciation and depletion	0	0	0	0
Accretion	0	0	0	0
(Gain) loss on derivative financial instruments	0	0	0	0
Selling, general and administrative expenses	0	0	0	0
Impairments	0	0	0	0
Other operating costs	0	0	0	0
Total costs and expenses	0	0	3	0
Operating income (loss)	0	0	(3)	0
Other income (expense)
Interest income	0	0	0	0
Interest expense	(195)	(119)	(590)	(291)
Other, net	54	29	183	112
Total other income (expense)	(141)	(90)	(407)	(179)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	(141)	(90)	(410)	(179)
Income tax benefit (expense)	0	0	0	0
Income (loss) from unconsolidated affiliates, net of tax	0	0	0	0
Income (loss) from consolidated affiliates, net of tax	0	0	0	0
Net income (loss)	(141)	(90)	(410)	(179)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	0	0	0	0
Postretirement medical plan change	0	0	0	0
Income tax on postretirement medical and pension changes	0	0	0	0
Other comprehensive income (loss)	0	0	0	0
Total comprehensive income (loss)	$ (141)	$ (90)	$ (410)	$ (179)