v3.26.1
Long-Term Debt - Additional Information (Details)
 1 Months Ended 3 Months Ended 6 Months Ended
May 20, 2026
USD ($)
Apr. 01, 2026
USD ($)
May 31, 2026
USD ($)
treasury_lock_agreement
Mar. 31, 2026
USD ($)
treasury_lock_agreement
Jun. 30, 2026
USD ($)
treasury_lock_agreement
Jun. 30, 2025
USD ($)
Jun. 30, 2026
USD ($)
treasury_lock_agreement
Jun. 30, 2025
USD ($)
Mar. 30, 2026
USD ($)
Jun. 29, 2023
USD ($)
Debt Instrument [Line Items]                    
Weighted average interest rates, as of December 31         1.33%   1.33%      
Debt instrument, face amount         $ 94,000,000   $ 94,000,000      
Proceeds from long-term debt, net of discount             1,293,000,000 $ 876,000,000    
Repayments of Long-Term Debt             $ 1,061,000,000 606,000,000    
Settled Debt Weighted Average Interest Rate         3.69   3.69      
Debt instrument, maturity date, Description             maturity dates ranging from 2031 through 2058      
Minimum                    
Debt Instrument [Line Items]                    
Debt Instrument, Interest Rate During Period             0.00%      
Settled Debt Interest Rate         0   0      
Maximum                    
Debt Instrument [Line Items]                    
Debt Instrument, Interest Rate During Period             1.74%      
Settled Debt Interest Rate         7.54   7.54      
Treasury lock agreements | Designated as Hedging Instrument                    
Debt Instrument [Line Items]                    
Weighted average interest rates, as of December 31         4.39%   4.39%      
Debt instrument, face amount         $ 75,000,000   $ 75,000,000   $ 600,000,000  
Derivative, number of instruments held | treasury_lock_agreement       10 3   3      
Debt instrument, term       10 years     7 years      
Gain (loss) from termination of derivative instruments       $ 3,000,000            
Ineffectiveness recognized on hedge instruments         $ 0 $ 0 $ 0 $ 0    
Treasury lock agreements | Designated as Hedging Instrument | Three Years [Member]                    
Debt Instrument [Line Items]                    
Debt instrument, face amount     $ 400,000,000              
Derivative, number of instruments held | treasury_lock_agreement     1              
Debt instrument, term     3 years              
Gain (loss) from termination of derivative instruments     $ 1,000,000              
Treasury lock agreements | Designated as Hedging Instrument | Thirty Years [Member]                    
Debt Instrument [Line Items]                    
Debt instrument, face amount     $ 175,000,000              
Derivative, number of instruments held | treasury_lock_agreement     5              
Debt instrument, term     30 years              
Gain (loss) from termination of derivative instruments     $ 7,000,000              
Senior notes—fixed rate | American Water Capital Corp. (AWCC)                    
Debt Instrument [Line Items]                    
Debt instrument, face amount $ 500,000,000 $ 700,000,000                
Interest rate 4.625%                  
Proceeds from long-term debt, net of discount $ 498,000,000                  
Accounts payable, interest-bearing, interest rate 3.625%                  
Senior notes—fixed rate | Senior Note 5.700% Due 2055 | American Water Capital Corp. (AWCC)                    
Debt Instrument [Line Items]                    
Interest rate   5.20%                
Proceeds from long-term debt, net of discount   $ 695,000,000                
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