Equity Method Investments, Fresh-Start Adjustment

No definition available.

Name:	aci_EquityMethodInvestmentsFreshStartAdjustment
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 35
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=99375318&loc=d3e32787-111569

+ Details

Name:	us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(12))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(10))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(f))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the investments in and advances to affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 6
-Subparagraph (SX 210.12-14.1(b))
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611322-123010

+ Details

Name:	us-gaap_InvestmentsInAndAdvancesToAffiliatesAtFairValuePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_InvestmentsInAndAdvancesToAffiliatesAtFairValueRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of a controlling interest in another entity or an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow associated with the sale of equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aci_MillenniumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aci_TongueRiverMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_PredecessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aci_KnightHawkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aci_DtaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aci_OtherEquityMethodInvestmentCompaniesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_SuccessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Equity Method Investments and Membership Interests in Joint Ventures (Schedule of Equity Method Investments) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended	12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Sep. 30, 2015	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2015
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					$ 96,074
Ending Balance	$ 96,074				106,107
Millennium
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Impairment of equity investment		$ (38,000)
Tongue River
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Impairment of equity investment			$ (21,300)
Predecessor
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	102,625			$ 201,877		$ 235,842
Advances to (distributions from) affiliates, net				(4,934)		(18,360)
Equity in comprehensive income (loss)				5,314		7,979
Impairment of equity investment				(38,025)		(21,325)
Sale of equity investment						(2,259)
Fresh start accounting adjustment				(61,607)
Ending Balance				102,625		201,877
Predecessor \| Knight Hawk
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	94,000			151,592		158,477
Advances to (distributions from) affiliates, net				(8,374)		(29,862)
Equity in comprehensive income (loss)				9,033		22,977
Fresh start accounting adjustment				(58,251)
Ending Balance				94,000		151,592
Predecessor \| DTA
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	8,600			13,239		13,738
Advances to (distributions from) affiliates, net				1,474		3,207
Equity in comprehensive income (loss)				(2,095)		(3,706)
Fresh start accounting adjustment				(4,018)
Ending Balance				8,600		13,239
Predecessor \| Millennium
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance				37,589		40,223
Advances to (distributions from) affiliates, net				1,966		7,052
Equity in comprehensive income (loss)				(1,530)		(9,686)
Impairment of equity investment				(38,025)
Fresh start accounting adjustment				0
Ending Balance						37,589
Predecessor \| Tongue River
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance						20,740
Advances to (distributions from) affiliates, net						913
Equity in comprehensive income (loss)						(328)
Impairment of equity investment						(21,325)
Fresh start accounting adjustment				0
Predecessor \| Other
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	25			(543)		2,664
Advances to (distributions from) affiliates, net						330
Equity in comprehensive income (loss)				(94)		(1,278)
Sale of equity investment						(2,259)
Fresh start accounting adjustment				662
Ending Balance				$ 25		$ (543)
Successor
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					96,074
Advances to (distributions from) affiliates, net	(8,254)				(5,722)
Equity in comprehensive income (loss)	1,703				8,597
Investments in affiliates					7,158
Ending Balance	96,074				106,107
Successor \| Knight Hawk
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					87,493
Advances to (distributions from) affiliates, net	(9,076)				(8,736)
Equity in comprehensive income (loss)	2,569				11,409
Ending Balance	87,493				90,166
Successor \| DTA
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					8,581
Advances to (distributions from) affiliates, net	822				3,014
Equity in comprehensive income (loss)	(841)				(2,812)
Investments in affiliates					7,158
Ending Balance	8,581				15,941
Successor \| Millennium
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					0
Advances to (distributions from) affiliates, net	0				0
Equity in comprehensive income (loss)	0				0
Ending Balance	0				0
Successor \| Tongue River
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					0
Advances to (distributions from) affiliates, net	0				0
Equity in comprehensive income (loss)	0				0
Ending Balance	0				0
Successor \| Other
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance					0
Advances to (distributions from) affiliates, net	0				0
Equity in comprehensive income (loss)	(25)				0
Ending Balance	$ 0				$ 0