| Document and Entity Information | Oct. 28, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001410636 |
| Document Type | 8-K |
| Document Period End Date | Oct. 28, 2021 |
| Entity Registrant Name | American Water Works Company, Inc. |
| Entity File Number | 001-34028 |
| Entity Incorporation State Country Code | DE |
| Entity Tax Identification Number | 51-0063696 |
| Entity Address, Address Line One | 1 Water Street |
| Entity Address, City or Town | Camden |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 08102-1658 |
| City Area Code | (856) |
| Local Phone Number | 955-4001 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, par value $0.01 per share |
| Trading Symbol | AWK |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
