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Long-Term Debt - Additional Information (Details)
 1 Months Ended 3 Months Ended 12 Months Ended
Aug. 08, 2025
USD ($)
Feb. 27, 2025
USD ($)
Jun. 29, 2023
USD ($)
$ / shares
Feb. 18, 2026
USD ($)
treasury_lock_agreement
Feb. 28, 2025
USD ($)
treasury_lock_agreement
Aug. 31, 2025
USD ($)
treasury_lock_agreement
Dec. 31, 2025
USD ($)
treasury_lock_agreement
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Debt Instrument [Line Items]                  
Long-term debt             $ 14,303,000,000 $ 13,193,000,000  
Line of credit facility consolidated debt to consolidated capitalization ratio             0.59    
Redeemable debt, amount outstanding             $ 773,000,000    
Weighted average interest rates, as of December 31             2.61%    
Debt issuance cost             $ 17,000,000    
Proceeds from long-term debt, net of discount             $ 1,781,000,000 1,437,000,000 $ 1,264,000,000
Treasury lock agreements | Designated as Hedging Instrument                  
Debt Instrument [Line Items]                  
Weighted average interest rates, as of December 31             4.47%    
Debt instrument, face amount         $ 500,000,000 $ 450,000,000 $ 200,000,000    
Derivative, number of instruments held | treasury_lock_agreement         10 11 6    
Debt instrument, term         10 years 30 years      
Gain (loss) from termination of derivative instruments         $ 3,000,000 $ 13,000,000      
Ineffectiveness recognized on hedge instruments             $ 0 $ 0 $ 0
Treasury lock agreements | Designated as Hedging Instrument | Subsequent Event                  
Debt Instrument [Line Items]                  
Weighted average interest rates, as of December 31       4.62%          
Debt instrument, face amount       $ 150,000,000          
Derivative, number of instruments held | treasury_lock_agreement       4          
Treasury lock agreements | Designated as Hedging Instrument | Minimum                  
Debt Instrument [Line Items]                  
Debt instrument, term             10 years    
Treasury lock agreements | Designated as Hedging Instrument | Minimum | Subsequent Event                  
Debt Instrument [Line Items]                  
Debt instrument, term       10 years          
Treasury lock agreements | Designated as Hedging Instrument | Maximum                  
Debt Instrument [Line Items]                  
Debt instrument, term             30 years    
Treasury lock agreements | Designated as Hedging Instrument | Maximum | Subsequent Event                  
Debt Instrument [Line Items]                  
Debt instrument, term       30 years          
American Water Capital Corp. (AWCC)                  
Debt Instrument [Line Items]                  
Weighted average interest rates, as of December 31             3.89% 4.65%  
Various Debt | American Water Capital Corp. (AWCC)                  
Debt Instrument [Line Items]                  
Debt instrument, redemption price, percentage     100.00%            
Line of credit                  
Debt Instrument [Line Items]                  
Line of credit facility consolidated debt to consolidated capitalization ratio             0.70    
Senior notes—fixed rate | Senior Note 5.700% Due 2055 | American Water Capital Corp. (AWCC)                  
Debt Instrument [Line Items]                  
Debt instrument, face amount $ 900,000,000                
Interest rate 5.70%                
Proceeds from long-term debt, net of discount $ 887,000,000                
Senior notes—fixed rate | Senior Note 5.250% Due 2035 | American Water Capital Corp. (AWCC)                  
Debt Instrument [Line Items]                  
Debt instrument, face amount   $ 800,000,000              
Interest rate   5.25%              
Proceeds from long-term debt, net of discount   $ 792,000,000              
Senior notes—fixed rate | Senior Note 3.400% Due 2025 | American Water Capital Corp. (AWCC)                  
Debt Instrument [Line Items]                  
Interest rate   3.40%              
Convertible Debt | Exchangeable Senior Note 3.625% Due 2026 | American Water Capital Corp. (AWCC)                  
Debt Instrument [Line Items]                  
Debt instrument, face amount     $ 1,035,000,000            
Interest rate     3.625%            
Proceeds from long-term debt, net of discount     $ 1,022,000,000            
Debt instrument, exchangeable, conversion ratio     0.0058213            
Debt instrument, exchangeable, conversion price (dollars per share) | $ / shares     $ 171.78            
Other American Water subsidiaries | Collateralized Debt Obligations | Private activity bonds and government funded debt—fixed rate                  
Debt Instrument [Line Items]                  
Long-term debt             $ 709,000,000