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Short-Term Debt - Schedule of Short-Term Borrowings Activity (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Short-term Debt [Line Items]    
Weighted average interest rates, as of December 31 2.61%  
American Water Capital Corp. (AWCC)    
Short-term Debt [Line Items]    
Average borrowings $ 1,170 $ 161
Maximum borrowings outstanding $ 1,595 $ 880
Weighted average interest rates, as of December 31 3.89% 4.65%