Reorganization items, net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Reorganization items, net	$ (3,875)	$ 0
Professional fee expense	(17,800)
Non-cash gains on rejected contracts	13,900
Payments for reorganization items	$ 1,500

Payments for Reorganization Items

No definition available.

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Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) on rejection of leases and other executory contracts, reported as a reorganization item.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=34712771&loc=d3e56145-112766

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 45
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=64805424&loc=d3e55730-112764

+ Details

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- References

No definition available.

+ Details

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- Definition

Amount of reorganization items related to legal and advisory professional fees related to entities not in bankruptcy.

+ References

+ Details

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

- Definition

Amount of gain (loss) on settlement of other claims for entities not in bankruptcy, reported as a reorganization item.

+ References

+ Details

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