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Long-Term Debt - Schedule of Issued (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2025
USD ($)
Debt Instrument [Line Items]  
Amount $ 1,789
AWCC, Senior Notes, Fixed Rate 5.70% | Senior notes—fixed rate | American Water Capital Corp. (AWCC)  
Debt Instrument [Line Items]  
Rate 5.70%
Amount $ 900
Other American Water Subsidiaries, Fixed Rate Ranging From 0.00% to 3.71% | Private activity bonds and government funded debt—fixed rate | Other American Water Subsidiaries  
Debt Instrument [Line Items]  
Rate 2.30%
Amount $ 89
Other American Water Subsidiaries, Fixed Rate Ranging From 0.00% to 3.71% | Private activity bonds and government funded debt—fixed rate | Other American Water Subsidiaries | Minimum  
Debt Instrument [Line Items]  
Rate 0.00%
Other American Water Subsidiaries, Fixed Rate Ranging From 0.00% to 3.71% | Private activity bonds and government funded debt—fixed rate | Other American Water Subsidiaries | Maximum  
Debt Instrument [Line Items]  
Rate 3.71%
AWCC, Senior Notes, Fixed Rate 5.25% | Senior notes—fixed rate | American Water Capital Corp. (AWCC)  
Debt Instrument [Line Items]  
Rate 5.25%
Amount $ 800