Supplemental Guarantor/Non-Guarantor Financial Information (Details 3) - USD ($) $ in Thousands	9 Months Ended		63 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Cash flows from continuing operations
Net cash provided by (used in) operating activities	$ 24,068	$ 57,032
Investing activities
Purchases of property, plant and equipment	(7,165)	(11,327)
Investment in development projects	(1,894)	(2,110)	$ (12,000)
Other	69	33
Net cash provided by (used in) investing activities	(8,990)	(13,404)
Financing activities
Repayment of senior notes	0	(62,094)
Principal payments on federal coal leases	(574)	0
Payment of debt refinancing costs	0	(408)
Payments of deferred financing costs	(936)	0
Payment amortized to deferred gain	(6,298)	(6,294)
Proceeds from issuance of common stock	0	68,850
Cash paid for equity offering	0	(4,490)
Other	(1,952)	(1,958)
Net cash provided by (used in) financing activities	(9,760)	(6,394)
Net increase (decrease) in cash, cash equivalents, and restricted cash	5,318	37,234
Cash, cash equivalents, and restricted cash at beginning of period	108,673	84,433
Cash, cash equivalents, and restricted cash at end of period	113,991	121,667	113,991
Parent Guarantor (CPE Inc.) | Reportable Legal Entities [Member]
Cash flows from continuing operations
Net cash provided by (used in) operating activities	0	0
Investing activities
Purchases of property, plant and equipment	0	0
Investment in development projects	0	0
Other	0	0
Net cash provided by (used in) investing activities	0	0
Financing activities
Repayment of senior notes		0
Principal payments on federal coal leases	0
Payment of debt refinancing costs		0
Payments of deferred financing costs	0
Payment amortized to deferred gain	0	0
Proceeds from issuance of common stock		0
Cash paid for equity offering		0
Other	0	0
Net cash provided by (used in) financing activities	0	0
Net increase (decrease) in cash, cash equivalents, and restricted cash	0	0
Cash, cash equivalents, and restricted cash at beginning of period	0	0
Cash, cash equivalents, and restricted cash at end of period	0	0	0
Issuers | Reportable Legal Entities [Member]
Cash flows from continuing operations
Net cash provided by (used in) operating activities	7,928	42,610
Investing activities
Purchases of property, plant and equipment	(114)	(1,420)
Investment in development projects	0	0
Other	13	0
Net cash provided by (used in) investing activities	(101)	(1,420)
Financing activities
Repayment of senior notes		(62,094)
Principal payments on federal coal leases	0
Payment of debt refinancing costs		0
Payments of deferred financing costs	0
Payment amortized to deferred gain	(6,298)	(6,294)
Proceeds from issuance of common stock		68,850
Cash paid for equity offering		(4,490)
Other	0	(23)
Net cash provided by (used in) financing activities	(6,298)	(4,051)
Net increase (decrease) in cash, cash equivalents, and restricted cash	1,529	37,139
Cash, cash equivalents, and restricted cash at beginning of period	107,818	83,673
Cash, cash equivalents, and restricted cash at end of period	109,347	120,812	109,347
Guarantor Subsidiaries | Reportable Legal Entities [Member]
Cash flows from continuing operations
Net cash provided by (used in) operating activities	11,397	14,014
Investing activities
Purchases of property, plant and equipment	(7,051)	(9,907)
Investment in development projects	(1,894)	(2,110)
Other	56	33
Net cash provided by (used in) investing activities	(8,889)	(11,984)
Financing activities
Repayment of senior notes		0
Principal payments on federal coal leases	(574)
Payment of debt refinancing costs		0
Payments of deferred financing costs	0
Payment amortized to deferred gain	0	0
Proceeds from issuance of common stock		0
Cash paid for equity offering		0
Other	(1,952)	(1,935)
Net cash provided by (used in) financing activities	(2,526)	(1,935)
Net increase (decrease) in cash, cash equivalents, and restricted cash	(18)	95
Cash, cash equivalents, and restricted cash at beginning of period	856	760
Cash, cash equivalents, and restricted cash at end of period	837	855	837
Non-Guarantor Subsidiaries | Reportable Legal Entities [Member]
Cash flows from continuing operations
Net cash provided by (used in) operating activities	4,743	408
Investing activities
Purchases of property, plant and equipment	0	0
Investment in development projects	0	0
Other	0	0
Net cash provided by (used in) investing activities	0	0
Financing activities
Repayment of senior notes		0
Principal payments on federal coal leases	0
Payment of debt refinancing costs		(408)
Payments of deferred financing costs	(936)
Payment amortized to deferred gain	0	0
Proceeds from issuance of common stock		0
Cash paid for equity offering		0
Other	0	0
Net cash provided by (used in) financing activities	(936)	(408)
Net increase (decrease) in cash, cash equivalents, and restricted cash	3,807	0
Cash, cash equivalents, and restricted cash at beginning of period	0	0
Cash, cash equivalents, and restricted cash at end of period	$ 3,807	$ 0	$ 3,807