Long-Term Debt Retired Through Sinking Fund Provisions, Optional Redemptions or Payments at Maturities (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Debt Instrument [Line Items]    
Total retirements and redemptions $ 20 $ 88
Debt retired during the year    
Debt Instrument [Line Items]    
Total retirements and redemptions $ 20  
Debt retired during the year | Other American Water subsidiaries | Mandatorily redeemable preferred stock    
Debt Instrument [Line Items]    
Interest Rate 8.49%  
Maturity Date, Maximum 2036  
Total retirements and redemptions $ 1  
Debt retired during the year | Fixed rate two | American Water Capital Corp. | Private activity bonds and government funded debt    
Debt Instrument [Line Items]    
Maturity Date, Minimum [1] 2021  
Maturity Date, Maximum [1] 2031  
Total retirements and redemptions [1] $ 1  
Debt retired during the year | Fixed rate two | Other American Water subsidiaries | Private activity bonds and government funded debt    
Debt Instrument [Line Items]    
Maturity Date, Minimum 2016  
Maturity Date, Maximum 2041  
Total retirements and redemptions $ 18  
Debt retired during the year | Fixed rate two | Minimum | American Water Capital Corp. | Private activity bonds and government funded debt    
Debt Instrument [Line Items]    
Interest Rate [1] 1.79%  
Debt retired during the year | Fixed rate two | Minimum | Other American Water subsidiaries | Private activity bonds and government funded debt    
Debt Instrument [Line Items]    
Interest Rate 0.00%  
Debt retired during the year | Fixed rate two | Maximum | American Water Capital Corp. | Private activity bonds and government funded debt    
Debt Instrument [Line Items]    
Interest Rate [1] 2.90%  
Debt retired during the year | Fixed rate two | Maximum | Other American Water subsidiaries | Private activity bonds and government funded debt    
Debt Instrument [Line Items]    
Interest Rate 5.30%  
[1] American Water Capital Corp., which is a wholly owned subsidiary of the Company, has a support agreement with the Company that, under certain circumstances, is the functional equivalent of a guarantee. This indebtedness is considered “debt” for purposes of this support agreement.