| Document and Entity Information | May 03, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | May 3, 2021 |
| Entity Registrant Name | AMERICAN STATES WATER CO |
| Entity Incorporation, State or Country Code | CA |
| Entity File Number | 001-14431 |
| Entity Tax Identification Number | 95-4676679 |
| Entity Address, Address Line One | 630 East Foothill Blvd. |
| Entity Address, City or Town | San Dimas |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 91773-1212 |
| City Area Code | 909 |
| Local Phone Number | 394-3600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | American States Water Company Common Shares |
| Trading Symbol | AWR |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001056903 |
| Amendment Flag | false |
