Supplemental Guarantor/Non-Guarantor Financial Information (Details 1) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	$ 233,080	$ 248,884	$ 655,087	$ 673,813
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	208,504	205,631	597,448	571,600
Depreciation and depletion	17,392	18,789	46,788	56,683
Accretion	1,947	1,865	5,359	5,532
(Gain) loss on derivative financial instruments	(730)	(838)	(730)	3,102
Selling, general and administrative expenses	5,101	13,059	25,393	33,861
Impairments	0	0	800	0
Other operating costs	149	121	331	429
Total costs and expenses	232,363	238,627	675,389	671,207
Operating income (loss)	717	10,257	(20,302)	2,606
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost	20,016	1,591	23,251	4,774
Interest income	304	147	841	304
Interest expense	(8,411)	(9,573)	(28,141)	(32,351)
Other, net	67	(98)	(535)	(546)
Total other income (expense)	11,976	(7,933)	(4,584)	(27,819)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	12,693	2,324	(24,886)	(25,213)
Income tax benefit (expense)	(5)	115	(302)	(36)
Income (loss) from unconsolidated affiliates, net of tax	3	138	269	771
Net income (loss)	12,691	2,577	(24,919)	(24,478)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(612)	(1,821)	(4,287)	(5,462)
Postretirement medical plan change	24,659	0	24,659	0
Postretirement medical plan termination	(19,477)	0	(19,477)	0
Income tax on postretirement medical plan	0	0	0	0
Other comprehensive income (loss)	4,570	(1,821)	895	(5,462)
Total comprehensive income (loss)	17,261	756	(24,024)	(29,940)
Eliminations
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	(1,650)	(2,238)	(6,053)
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)			0
Depreciation and depletion			0
Accretion			0
Selling, general and administrative expenses	(1,650)	(2,238)	(6,053)
Other operating costs			0
Total costs and expenses	(1,650)	(2,238)	(6,053)
Operating income (loss)	0		0
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost			0
Interest income			0
Interest expense			0
Other, net			0
Total other income (expense)			0
Income (loss) before income tax provision and earnings from unconsolidated affiliates			0
Income tax benefit (expense)			0
Income (loss) from unconsolidated affiliates, net of tax			0
Income (loss) from consolidated affiliates, net of tax	(33,342)	(24,735)	6,973
Net income (loss)	(33,342)	(24,735)	6,973
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	1,224	3,642	8,574
Postretirement medical plan change	(49,318)		(49,318)
Postretirement medical plan termination	38,954		38,954
Income tax on postretirement medical plan	0	0	0
Other comprehensive income (loss)	(9,140)	3,642	(1,790)
Total comprehensive income (loss)	(42,482)	(21,093)	5,183
Parent Guarantor (CPE Inc.) | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	1,650	2,238	6,053	5,887
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
Other operating costs	0	0	0	0
Total costs and expenses	0	0	0	0
Operating income (loss)	1,650	2,238	6,053	5,887
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost	0	0	0	0
Interest income	0	0	2	1
Interest expense	0	0	0	(48)
Other, net	0	0	0	0
Total other income (expense)	0	0	2	(47)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	1,650	2,238	6,055	5,840
Income tax benefit (expense)	(5)	115	(302)	(36)
Income (loss) from unconsolidated affiliates, net of tax	0	0	0	0
Income (loss) from consolidated affiliates, net of tax	11,046	224	(30,673)	(30,282)
Net income (loss)	12,691	2,577	(24,919)	(24,478)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(612)	(1,821)	(4,287)	(5,462)
Postretirement medical plan change	24,659		24,659
Postretirement medical plan termination	(19,477)		(19,477)
Income tax on postretirement medical plan	0	0	0	0
Other comprehensive income (loss)	4,570	(1,821)	895	(5,462)
Total comprehensive income (loss)	17,261	756	(24,024)	(29,940)
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)	11	2	3	9
Depreciation and depletion	305	245	859	626
Accretion	0	0	0	0
(Gain) loss on derivative financial instruments	0	0	0	0
Selling, general and administrative expenses	6,751	15,297	31,446	39,748
Impairments	0		0
Other operating costs	0	0	0	23
Total costs and expenses	7,067	15,544	32,308	40,406
Operating income (loss)	(7,067)	(15,544)	(32,308)	(40,406)
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost	3,354	261	3,894	784
Interest income	301	147	836	303
Interest expense	(7,975)	(9,243)	(27,014)	(31,309)
Other, net	(101)	(54)	(323)	(183)
Total other income (expense)	(4,421)	(8,889)	(22,607)	(30,405)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	(11,488)	(24,433)	(54,915)	(70,811)
Income tax benefit (expense)	0	0	0	0
Income (loss) from unconsolidated affiliates, net of tax	6	5	15	15
Income (loss) from consolidated affiliates, net of tax	22,528	24,652	24,228	40,514
Net income (loss)	11,046	224	(30,673)	(30,282)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(612)	(1,821)	(4,287)	(5,462)
Postretirement medical plan change	24,659		24,659
Postretirement medical plan termination	(19,477)		(19,477)
Income tax on postretirement medical plan	0	0	0	0
Other comprehensive income (loss)	4,570	(1,821)	895	(5,462)
Total comprehensive income (loss)	15,616	(1,597)	(29,778)	(35,744)
Guarantor Subsidiaries | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	233,080	248,884	655,087	673,813
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	208,493	205,629	597,404	571,588
Depreciation and depletion	17,087	18,544	45,929	56,057
Accretion	1,947	1,865	5,359	5,532
(Gain) loss on derivative financial instruments	(730)	(838)	(730)	3,102
Selling, general and administrative expenses	0	0	0	0
Impairments	0		800
Other operating costs	149	121	331	406
Total costs and expenses	226,946	225,321	649,093	636,685
Operating income (loss)	6,134	23,563	5,994	37,128
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost	16,662	1,330	19,357	3,990
Interest income	3	0	3	0
Interest expense	(103)	(135)	(317)	(404)
Other, net	67	(98)	(535)	(546)
Total other income (expense)	16,629	1,097	18,508	3,040
Income (loss) before income tax provision and earnings from unconsolidated affiliates	22,763	24,660	24,502	40,168
Income tax benefit (expense)	0	0	0	0
Income (loss) from unconsolidated affiliates, net of tax	(3)	133	254	756
Income (loss) from consolidated affiliates, net of tax	(232)	(141)	(528)	(410)
Net income (loss)	22,528	24,652	24,228	40,514
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(612)	(1,821)	(4,287)	(5,462)
Postretirement medical plan change	24,659		24,659
Postretirement medical plan termination	(19,477)		(19,477)
Income tax on postretirement medical plan	0	0	0	0
Other comprehensive income (loss)	4,570	(1,821)	895	(5,462)
Total comprehensive income (loss)	27,098	22,831	25,123	35,052
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	41	3
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
Other operating costs	0	0	0	0
Total costs and expenses	0	0	41	3
Operating income (loss)	0	0	(41)	(3)
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost	0	0	0	0
Interest income	0	0	0	0
Interest expense	(333)	(195)	(810)	(590)
Other, net	101	54	323	183
Total other income (expense)	(232)	(141)	(487)	(407)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	(232)	(141)	(528)	(410)
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)	(232)	(141)	(528)	(410)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	0	0	0	0
Postretirement medical plan change	0		0
Postretirement medical plan termination	0		0
Income tax on postretirement medical plan	0	0	0	0
Other comprehensive income (loss)	0	0	0	0
Total comprehensive income (loss)	$ (232)	$ (141)	$ (528)	$ (410)