| Document and Entity Information Document | Apr. 08, 2021 |
| Document Information [Line Items] | |
| Document Period End Date | Apr. 8, 2021 |
| Entity Registrant Name | American Water Works Company, Inc. |
| Document Type | 8-K |
| Document Period End Date | Apr. 8, 2021 |
| Entity File Number | 001-34028 |
| Entity Incorporation, State or 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 |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001410636 |
| Amendment Flag | false |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | AWK |
| Security Exchange Name | NYSE |
