Workers' Compensation Expense (Narrative) (Details) $ in Millions	1 Months Ended
Feb. 29, 2020 USD ($)
Workers' Compensation Expense
Additional collateral amount	$ 71.1
Number of days required to submit additional collateral to retain self-insurance status	30 days

Contingent amount to be paid to retain the self - insure status within specified time limit of 30 days from receipt of letter.

No definition available.

Name:	aci_AdditionalCollateralContingentAmountSelfInsured
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant