Supplemental Guarantor/Non-Guarantor Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	$ 260,737	$ 301,673	$ 244,148	$ 317,553	$ 341,791	$ 342,337	$ 320,850	$ 319,066	$ 1,124,111	$ 1,324,044	$ 1,396,097
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion and accretion, shown separately)									950,580	1,094,257	1,136,318
Depreciation and depletion									66,064	112,022	100,523
Amortization of port access rights									3,710	0	0
Accretion									12,555	15,136	15,342
(Gain) loss on derivative financial instruments									30,635	(7,805)	(25,611)
Selling, general and administrative expenses									48,925	50,201	53,066
Impairments									91,541	0	0
Other operating costs									1,492	2,693	4,077
Total costs and expenses									1,205,502	1,266,504	1,283,715
Gain on sale of Decker Mine						(74,300)			0	(74,262)	0
Operating income (loss)	(56,022)	17,458	(50,812)	7,984	22,407	85,844	7,428	16,124	(81,391)	131,802	112,382
Other income (expense)
Interest income									170	259	440
Interest expense									(47,561)	(77,160)	(41,665)
Tax agreement benefit (expense)									0	58,595	(10,515)
Other, net									62	(202)	2,423
Total other income (expense)									(47,329)	(18,508)	(49,317)
Income (loss) before income tax provision and earnings from unconsolidated affiliates									(128,720)	113,294	63,065
Income tax benefit (expense)									(77,380)	(34,913)	(11,629)
Earnings from unconsolidated affiliates, net of tax									1,200	579	535
Net income (loss)	$ (156,196)	$ 8,873	$ (52,897)	$ (4,680)	$ 5,665	$ 91,069	$ (2,148)	$ (15,626)	(204,900)	78,960	51,971
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service cost									1,252	989	1,775
Postretirement medical plan adjustment									(3,874)	(5,564)	10,824
Decker pension adjustments									0	0	3,199
Write-off of prior service costs related to Decker pension									0	3,183	0
Income tax on postretirement medical and pension adjustments									970	372	(5,616)
Other comprehensive income (loss)									(1,652)	(1,020)	10,182
Total comprehensive income (loss)									(206,552)	77,940	62,153
Eliminations
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									(7,308)	(8,796)	(9,535)
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 port access rights									0	0	0
Accretion									0	0	0
(Gain) loss on derivative financial instruments									0	0	0
Selling, general and administrative expenses									(7,308)	(8,796)	(9,535)
Impairments									0	0	0
Other operating costs									(1)	0	0
Total costs and expenses									(7,309)	(8,796)	(9,535)
Gain on sale of Decker Mine									0	0	0
Operating income (loss)									1	0	0
Other income (expense)
Interest income									0	0	0
Interest expense									0	0	0
Tax agreement benefit (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									1	0	0
Income tax benefit (expense)									0	0	0
Earnings from unconsolidated affiliates, net of tax									1	0	0
Earnings (losses) from consolidated affiliates, net of tax									213,788	(180,883)	(191,988)
Net income (loss)									213,790	(180,883)	(191,988)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service cost									(2,504)	(1,978)	(3,550)
Postretirement medical plan adjustment									7,748	11,128	(21,648)
Decker pension adjustments											(9,597)
Write-off of prior service costs related to Decker pension										(9,549)
Income tax on postretirement medical and pension adjustments									(1,940)	402	12,417
Other comprehensive income (loss)									3,304	3	(22,378)
Total comprehensive income (loss)									217,094	(180,880)	(214,366)
Parent Guarantor (CPE Inc.)
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									7,308	8,796	9,535
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 port access rights									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
Other operating costs									0	0	0
Total costs and expenses									0	0	0
Gain on sale of Decker Mine									0	0	0
Operating income (loss)									7,308	8,796	9,535
Other income (expense)
Interest income									0	0	0
Interest expense									(36)	0	0
Tax agreement benefit (expense)									0	58,595	(10,515)
Other, net									0	0	(226)
Total other income (expense)									(36)	58,595	(10,741)
Income (loss) before income tax provision and earnings from unconsolidated affiliates									7,272	67,391	(1,207)
Income tax benefit (expense)									(62,019)	(20,439)	3,850
Earnings from unconsolidated affiliates, net of tax									0	0	0
Earnings (losses) from consolidated affiliates, net of tax									(150,153)	32,007	49,328
Net income (loss)									(204,900)	78,960	51,971
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service cost									1,252	989	1,775
Postretirement medical plan adjustment									(3,874)	(5,564)	10,824
Decker pension adjustments											3,199
Write-off of prior service costs related to Decker pension										3,183
Income tax on postretirement medical and pension adjustments									970	372	(5,616)
Other comprehensive income (loss)									(1,652)	(1,020)	10,182
Total comprehensive income (loss)									(206,552)	77,940	62,153
Issuers
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									0	0	13
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion and accretion, shown separately)									(32)	260	87
Depreciation and depletion									2,127	2,994	2,679
Amortization of port access rights									0	0	0
Accretion									0	0	0
(Gain) loss on derivative financial instruments									0	0	0
Selling, general and administrative expenses									56,233	58,997	62,600
Impairments									0	0	0
Other operating costs									0	0	3,067
Total costs and expenses									58,328	62,252	68,433
Gain on sale of Decker Mine									0	0	0
Operating income (loss)									(58,328)	(62,252)	(68,420)
Other income (expense)
Interest income									169	259	440
Interest expense									(43,638)	(68,064)	(39,410)
Tax agreement benefit (expense)									0	0	0
Other, net									(177)	(999)	(395)
Total other income (expense)									(43,646)	(68,804)	(39,365)
Income (loss) before income tax provision and earnings from unconsolidated affiliates									(101,974)	(131,056)	(107,786)
Income tax benefit (expense)									15,339	11,909	15,645
Earnings from unconsolidated affiliates, net of tax									11	(4)	23
Earnings (losses) from consolidated affiliates, net of tax									(63,530)	151,158	141,446
Net income (loss)									(150,153)	32,007	49,328
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service cost									1,252	989	1,775
Postretirement medical plan adjustment									(3,874)	(5,564)	10,824
Decker pension adjustments											3,199
Write-off of prior service costs related to Decker pension										3,183
Income tax on postretirement medical and pension adjustments									970	372	(5,616)
Other comprehensive income (loss)									(1,652)	(1,020)	10,182
Total comprehensive income (loss)									(151,805)	30,987	59,510
Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									1,124,111	1,308,391	1,374,610
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion and accretion, shown separately)									950,612	1,076,154	1,113,988
Depreciation and depletion									63,937	110,196	102,329
Amortization of port access rights									3,710	0	0
Accretion									12,555	12,333	11,328
(Gain) loss on derivative financial instruments									30,635	(7,805)	(25,611)
Selling, general and administrative expenses									0	0	0
Impairments									91,541	0	0
Other operating costs									1,493	2,693	1,010
Total costs and expenses									1,154,483	1,193,571	1,203,044
Gain on sale of Decker Mine									0	(74,262)	0
Operating income (loss)									(30,372)	189,081	171,566
Other income (expense)
Interest income									1	0	0
Interest expense									(3,543)	(8,710)	(1,890)
Tax agreement benefit (expense)									0	0	0
Other, net									62	374	2,516
Total other income (expense)									(3,480)	(8,336)	626
Income (loss) before income tax provision and earnings from unconsolidated affiliates									(33,851)	180,746	172,193
Income tax benefit (expense)									(30,762)	(27,889)	(32,473)
Earnings from unconsolidated affiliates, net of tax									1,188	584	512
Earnings (losses) from consolidated affiliates, net of tax									(105)	(2,282)	1,214
Net income (loss)									(63,530)	151,158	141,446
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service cost									1,252	989	1,775
Postretirement medical plan adjustment									(3,874)	(5,564)	10,824
Decker pension adjustments											3,199
Write-off of prior service costs related to Decker pension										3,183
Income tax on postretirement medical and pension adjustments									970	372	(5,616)
Other comprehensive income (loss)									(1,652)	(1,020)	10,182
Total comprehensive income (loss)									(65,182)	150,138	151,628
Non-Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									0	15,653	21,474
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion and accretion, shown separately)									0	17,841	22,243
Depreciation and depletion									0	(1,168)	(4,485)
Amortization of port access rights									0	0	0
Accretion									0	2,803	4,014
(Gain) loss on derivative financial instruments									0	0	0
Selling, general and administrative expenses									0	0	0
Impairments									0	0	0
Other operating costs									0	0	0
Total costs and expenses									0	19,476	21,772
Gain on sale of Decker Mine									0	0	0
Operating income (loss)									0	(3,823)	(298)
Other income (expense)
Interest income									0	0	0
Interest expense									(344)	(386)	(365)
Tax agreement benefit (expense)									0	0	0
Other, net									177	422	529
Total other income (expense)									(167)	36	164
Income (loss) before income tax provision and earnings from unconsolidated affiliates									(167)	(3,787)	(134)
Income tax benefit (expense)									62	1,505	1,348
Earnings from unconsolidated affiliates, net of tax									0	0	0
Earnings (losses) from consolidated affiliates, net of tax									0	0	0
Net income (loss)									(105)	(2,282)	1,214
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service cost									0	0	0
Postretirement medical plan adjustment									0	0	0
Decker pension adjustments											3,199
Write-off of prior service costs related to Decker pension										3,183
Income tax on postretirement medical and pension adjustments									0	(1,146)	(1,185)
Other comprehensive income (loss)									0	2,037	2,014
Total comprehensive income (loss)									$ (105)	$ (245)	$ 3,228