Subsequent Events (New First Lien Debt Facility) (Details) - USD ($)	Oct. 05, 2016	Jan. 21, 2016
London Interbank Offered Rate (LIBOR) [Member]
Subsequent Event [Line Items]
Basis spread on variable rate (percent)		9.00%
Base Rate [Member]
Subsequent Event [Line Items]
Basis spread on variable rate (percent)		8.00%
Subsequent Event [Member]
Subsequent Event [Line Items]
Holders of allowed claims percentage of common stock outstanding upon reorganization	94.00%
Subsequent Event [Member] \| New First Lien Debt Facility [Member] \| Senior Notes [Member]
Subsequent Event [Line Items]
Amount of debt instrument	$ 326,500,000.0
Term of instrument	5 years
Maximum amount of Debtors' liquidity	$ 300,000,000
Quarterly amortization payments	$ 816,250
Accrual of PIK interest (percent)	1.00%
Amount of voting equity interests of domestic subsidiaries guaranteed (percent)	100.00%
Amount of voting equity interests of foreign owned subsidiaries guaranteed (percent)	65.00%
Net cash proceeds of from debt issuances and other non-ordinary sales or dispositions (percent)	100.00%
Default to indebtedness	$ 35,000,000
Default to surety, reclamation or similar bond	50,000,000
Default to uninsured judgment	35,000,000
Uninsured losses or proceedings against	$ 35,000,000
Subsequent Event [Member] \| New First Lien Debt Facility [Member] \| Senior Notes [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Subsequent Event [Line Items]
Basis spread on variable rate (percent)	9.00%
Subsequent Event [Member] \| New First Lien Debt Facility [Member] \| Senior Notes [Member] \| Base Rate [Member]
Subsequent Event [Line Items]
Basis spread on variable rate (percent)	8.00%
Subsequent Event [Member] \| New First Lien Debt Facility [Member] \| Senior Notes [Member] \| Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Subsequent Event [Line Items]
Basis spread on variable rate (percent)	1.00%

Debt Instrument, Amortization Payments, Quarterly Amount

No definition available.

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- Definition

Debt Instrument, Covenant Compliance, Liquidity Compliance, Amount

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Covenants, Default to Indebtedness, Minimum

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenants, Default to Surety, Reclamation or Similar Bond, Minimum

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenants, Default to Uninsured Judgment, Minimum

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenants, Pre-Payment, Cash Proceeds of Debt and Other Non-Ordinary Sales or Dispositions, Percent

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenants, Uninsured Losses or Proceedings Against, Minimum

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Debt Instrument, Guarantees, Equity Interest of Domestic Subsidiaries, Percent

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Debt Instrument, Guarantees, Equity Interest of Foreign Subsidiaries, Percent

+ References

No definition available.

+ Details

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Namespace Prefix:	aci_
Data Type:	num:percentItemType
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Period Type:	instant

- Definition

Debt Instrument, Paid-in-Kind Interest, Accrual Rate, Percent

+ References

No definition available.

+ Details

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Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Plan of Reorganization, Terms of Plan, Holders of Allowed Claims, Percentage of Common Stock Outstanding Upon Reorganization

+ References

No definition available.

+ Details

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Namespace Prefix:	aci_
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Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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