| Cover | Feb. 06, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 6, 2020 |
| Entity File Number | 1-13105 |
| Entity Registrant Name | Arch Coal, Inc. |
| Entity Central Index Key | 0001037676 |
| Entity Tax Identification Number | 43-0921172 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | CityPlace One |
| Entity Address, Address Line Two | One CityPlace Drive, Suite 300 |
| Entity Address, City or Town | St. Louis |
| Entity Address, State or Province | MO |
| Entity Address, Postal Zip Code | 63141 |
| City Area Code | 314 |
| Local Phone Number | 994-2700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $.01 par value |
| Trading Symbol | ARCH |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
