Supplemental Guarantor/Non-Guarantor Financial Information (Details 1) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Oct. 17, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue		$ 213,893	$ 248,884	$ 229,201	$ 195,728	$ 227,928	$ 217,073	$ 174,188	$ 181,249	$ 887,706	$ 800,438	$ 1,124,111
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										752,715	646,404	950,580
Depreciation and depletion										72,270	27,218	66,064
Amortization of Intangible Assets										0	0	3,710
Accretion										7,072	6,645	12,555
(Gain) loss on derivative financial instruments										2,672	(8,180)	30,635
Selling, general and administrative expenses										47,482	50,868	48,925
Impairments									4,200	0	4,609	91,541
Debt restructuring costs	$ 4,700					200	4,500			23	4,665	0
Other operating costs										532	941	1,492
Total costs and expenses										882,766	733,170	1,205,502
Operating income (loss)		(2,441)	11,848	2,484	(6,951)	36,131	10,861	45,920	(25,645)	4,940	67,268	(81,391)
Other income (expense)
Interest income										485	138	170
Interest expense										(41,362)	(47,434)	(47,561)
Other, net										(885)	(1,001)	62
Total other income (expense)										(41,762)	(48,297)	(47,329)
Income (loss) before income tax provision and earnings from unconsolidated affiliates										(36,822)	18,971	(128,720)
Income tax benefit (expense)										29,470	2,213	(77,380)
Income (loss) from unconsolidated affiliates, net of tax										713	657	1,200
Net income (loss)		$ 17,839	$ 2,577	$ (6,948)	$ (20,108)	$ 24,511	$ (1,584)	$ 35,289	$ (36,375)	(6,639)	21,841	(204,900)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs										(7,283)	(5,253)	1,252
Postretirement medical plan adjustments										(794)	(1,792)	(3,874)
Postretirement medical plan change										0	42,851	0
Income tax on postretirement medical and pension changes										0	(971)	970
Other comprehensive income (loss)										(8,077)	34,835	(1,652)
Total comprehensive income (loss)										(14,716)	56,676	(206,552)
Eliminations
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue										(7,721)	(8,274)	(7,308)
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										0	0	0
Depreciation and depletion										0	0	0
Amortization of Intangible Assets										0	0	0
Accretion										0	0	0
(Gain) loss on derivative financial instruments										0	0	0
Selling, general and administrative expenses										(7,721)	(8,274)	(7,308)
Impairments										0	0	0
Debt restructuring costs										0	0
Other operating costs										0	0	(1)
Total costs and expenses										(7,721)	(8,274)	(7,309)
Operating income (loss)										0	0	1
Other income (expense)
Interest income										0	0	0
Interest expense										0	0	0
Other, net										0	0	0
Total other income (expense)										0	0	0
Income (loss) before income tax provision and earnings from unconsolidated affiliates										0	0	1
Income tax benefit (expense)										0	0	0
Income (loss) from unconsolidated affiliates, net of tax										0	0	1
Earnings (losses) from consolidated affiliates, net of tax										(64,353)	(137,772)	213,788
Net income (loss)										(64,353)	(137,772)	213,790
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs										14,566	10,506	(2,504)
Postretirement medical plan adjustments										1,588	3,584	7,748
Postretirement medical plan change										0	(85,702)
Income tax on postretirement medical and pension changes										0	1,942	(1,940)
Other comprehensive income (loss)										16,154	(69,670)	3,304
Total comprehensive income (loss)										(48,199)	(207,442)	217,092
Parent Guarantor (CPE Inc.) | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue										7,721	8,274	7,308
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										0	0	0
Depreciation and depletion										0	0	0
Amortization of Intangible Assets										0	0	0
Accretion										0	0	0
(Gain) loss on derivative financial instruments										0	0	0
Selling, general and administrative expenses										0	0	0
Impairments										0	0	0
Debt restructuring costs										0	0
Other operating costs										0	0	0
Total costs and expenses										0	0	0
Operating income (loss)										7,721	8,274	7,308
Other income (expense)
Interest income										1	27	0
Interest expense										(47)	(244)	(36)
Other, net										0	0	0
Total other income (expense)										(46)	(217)	(36)
Income (loss) before income tax provision and earnings from unconsolidated affiliates										7,675	8,057	7,272
Income tax benefit (expense)										(30,244)	(6,278)	(62,019)
Income (loss) from unconsolidated affiliates, net of tax										0	0	0
Earnings (losses) from consolidated affiliates, net of tax										15,930	20,062	(150,153)
Net income (loss)										(6,639)	21,841	(204,900)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs										(7,283)	(5,253)	1,252
Postretirement medical plan adjustments										(794)	(1,792)	(3,874)
Postretirement medical plan change										0	42,851
Income tax on postretirement medical and pension changes										0	(971)	970
Other comprehensive income (loss)										(8,077)	34,835	(1,652)
Total comprehensive income (loss)										(14,716)	56,676	(206,552)
Issuers | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue										0	0	0
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										13	122	(32)
Depreciation and depletion										897	1,172	2,127
Amortization of Intangible Assets										0	0	0
Accretion										0	0	0
(Gain) loss on derivative financial instruments										0	0	0
Selling, general and administrative expenses										55,203	59,142	56,233
Impairments										0	2,048	0
Debt restructuring costs										23	4,665
Other operating costs										0	0	0
Total costs and expenses										56,136	67,149	58,328
Operating income (loss)										(56,136)	(67,149)	(58,328)
Other income (expense)
Interest income										484	110	169
Interest expense										(39,970)	(46,318)	(43,638)
Other, net										(227)	(138)	(177)
Total other income (expense)										(39,713)	(46,346)	(43,646)
Income (loss) before income tax provision and earnings from unconsolidated affiliates										(95,849)	(113,495)	(101,974)
Income tax benefit (expense)										62,757	15,586	15,339
Income (loss) from unconsolidated affiliates, net of tax										13	16	11
Earnings (losses) from consolidated affiliates, net of tax										49,009	117,955	(63,530)
Net income (loss)										15,930	20,062	(150,153)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs										(7,283)	(5,253)	1,252
Postretirement medical plan adjustments										(794)	(1,792)	(3,874)
Postretirement medical plan change										0	42,851
Income tax on postretirement medical and pension changes										0	(971)	970
Other comprehensive income (loss)										(8,077)	34,835	(1,652)
Total comprehensive income (loss)										7,853	54,897	(151,805)
Guarantor Subsidiaries | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue										887,706	800,438	1,124,111
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										752,697	646,282	950,612
Depreciation and depletion										71,373	26,046	63,937
Amortization of Intangible Assets										0	0	3,710
Accretion										7,072	6,645	12,555
(Gain) loss on derivative financial instruments										2,672	(8,180)	30,635
Selling, general and administrative expenses										0	0	0
Impairments										0	2,561	91,541
Debt restructuring costs										0	0
Other operating costs										532	941	1,493
Total costs and expenses										834,346	674,295	1,154,483
Operating income (loss)										53,360	126,143	(30,372)
Other income (expense)
Interest income										0	1	1
Interest expense										(536)	(489)	(3,543)
Other, net										(886)	(1,001)	62
Total other income (expense)										(1,422)	(1,489)	(3,480)
Income (loss) before income tax provision and earnings from unconsolidated affiliates										51,938	124,654	(33,851)
Income tax benefit (expense)										(3,043)	(7,095)	(30,762)
Income (loss) from unconsolidated affiliates, net of tax										700	641	1,188
Earnings (losses) from consolidated affiliates, net of tax										(586)	(245)	(105)
Net income (loss)										49,009	117,955	(63,530)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs										(7,283)	(5,253)	1,252
Postretirement medical plan adjustments										(794)	(1,792)	(3,874)
Postretirement medical plan change										0	42,851
Income tax on postretirement medical and pension changes										0	(971)	970
Other comprehensive income (loss)										(8,077)	34,835	(1,652)
Total comprehensive income (loss)										40,932	152,790	(65,182)
Non-Guarantor Subsidiaries | Reportable Legal Entities [Member]
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue										0	0	0
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										5	0	0
Depreciation and depletion										0	0	0
Amortization of Intangible Assets										0	0	0
Accretion										0	0	0
(Gain) loss on derivative financial instruments										0	0	0
Selling, general and administrative expenses										0	0	0
Impairments										0	0	0
Debt restructuring costs										0	0
Other operating costs										0	0	0
Total costs and expenses										5	0	0
Operating income (loss)										(5)	0	0
Other income (expense)
Interest income										0	0	0
Interest expense										(809)	(383)	(344)
Other, net										228	138	177
Total other income (expense)										(581)	(245)	(167)
Income (loss) before income tax provision and earnings from unconsolidated affiliates										(586)	(245)	(167)
Income tax benefit (expense)										0	0	62
Income (loss) from unconsolidated affiliates, net of tax										0	0	0
Earnings (losses) from consolidated affiliates, net of tax										0	0	0
Net income (loss)										(586)	(245)	(105)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs										0	0	0
Postretirement medical plan adjustments										0	0	0
Postretirement medical plan change										0	0
Income tax on postretirement medical and pension changes										0	0	0
Other comprehensive income (loss)										0	0	0
Total comprehensive income (loss)										$ (586)	$ (245)	$ (105)