v3.26.1
Long-Term Debt (Details)
 1 Months Ended 3 Months Ended 6 Months Ended
May 20, 2026
USD ($)
Apr. 01, 2026
USD ($)
Mar. 31, 2026
USD ($)
treasury_lock_agreement
Jun. 29, 2023
USD ($)
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 ($)
treasury_lock_agreement
Jun. 30, 2025
USD ($)
Dec. 31, 2025
Debt Instrument [Line Items]                      
Proceeds from long-term debt, net of discount                 $ 1,293,000,000 $ 876,000,000  
Treasury lock agreements, Designated as Hedging Instrument | Designated as Hedging Instrument                      
Debt Instrument [Line Items]                      
Debt instrument, face amount             $ 75,000,000   $ 75,000,000    
Weighted average interest rates             4.39%   4.39%    
Derivative, number of instruments held | treasury_lock_agreement             3   3    
Debt instrument, term                 7 years    
Treasury lock agreements, Designated as Hedging Instrument, Terminated, Transaction 1 | Designated as Hedging Instrument                      
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            
Debt instrument, amoritazation period         3 years            
Treasury lock agreements, Designated as Hedging Instrument, Terminated, Transaction 1 | Designated as Hedging Instrument | Location, Statement of Income, Balance [Axis]: us-gaap:InterestExpenseNonoperating                      
Debt Instrument [Line Items]                      
Gain from termination of derivative instruments         $ 1,000,000            
Treasury lock agreements, Designated as Hedging Instrument, Terminated, Transaction 2 | Designated as Hedging Instrument                      
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            
Treasury lock agreements, Designated as Hedging Instrument, Terminated, Transaction 2 | Designated as Hedging Instrument | Location, Statement of Income, Balance [Axis]: us-gaap:OtherNonoperatingIncomeExpense                      
Debt Instrument [Line Items]                      
Gain from termination of derivative instruments         $ 7,000,000            
Treasury lock agreements, Designated as Hedging Instrument, Terminated | Designated as Hedging Instrument                      
Debt Instrument [Line Items]                      
Debt instrument, face amount     $ 600,000,000     $ 600,000,000          
Derivative, number of instruments held | treasury_lock_agreement     10     10          
Debt instrument, term           10 years          
Gain from termination of derivative instruments           $ 3,000,000          
Treasury Lock Agreements | Designated as Hedging Instrument                      
Debt Instrument [Line Items]                      
Ineffectiveness recognized on hedge instruments             $ 0 $ 0 $ 0 $ 0  
American Water Capital Corp.                      
Debt Instrument [Line Items]                      
Weighted average interest rates             3.99%   3.99%   3.89%
Senior Note 4.625% Due 2029 | Senior note - fixed rate | American Water Capital Corp.                      
Debt Instrument [Line Items]                      
Debt instrument, face amount $ 500,000,000                    
Interest rate 4.625%                    
Proceeds from long-term debt, net of discount $ 498,000,000                    
Exchangeable Senior Note 3.625% Due 2026 | Exchangeable Debt | American Water Capital Corp.                      
Debt Instrument [Line Items]                      
Interest rate 3.625%                    
Debt instrument, exchangeable, conversion ratio     0.005827 0.0058213              
Senior Note 5.200% Due 2036 | Senior note - fixed rate | American Water Capital Corp.                      
Debt Instrument [Line Items]                      
Debt instrument, face amount   $ 700,000,000                  
Interest rate   5.20%                  
Proceeds from long-term debt, net of discount   $ 695,000,000                  
Private activity bonds and government funded debt                      
Debt Instrument [Line Items]                      
Debt instrument, face amount             $ 94,000,000   $ 94,000,000    
Private activity bonds and government funded debt | Minimum                      
Debt Instrument [Line Items]                      
Interest rate             0.00%   0.00%    
Private activity bonds and government funded debt | Maximum                      
Debt Instrument [Line Items]                      
Interest rate             1.74%   1.74%    
Various Debt Maturing in 2029 Through 2061                      
Debt Instrument [Line Items]                      
Interest rate             1.33%   1.33%    
Repayments of debt                 $ 1,061,000,000    
Weighted average interest rates             3.69%   3.69%    
Various Debt Maturing in 2029 Through 2061 | Minimum                      
Debt Instrument [Line Items]                      
Interest rate             0.00%   0.00%    
Various Debt Maturing in 2029 Through 2061 | Maximum                      
Debt Instrument [Line Items]                      
Interest rate             7.54%   7.54%    
Exchangeable Notes | Exchangeable Debt | American Water Capital Corp.                      
Debt Instrument [Line Items]                      
Debt instrument, face amount       $ 1,035,000,000