Commitments and Contingencies - Additional Information (Details)
 1 Months Ended 9 Months Ended
Sep. 16, 2016
Complaint
Mar. 29, 2016
USD ($)
Dec. 17, 2015
Claim
Jun. 03, 2015
Claim
Jan. 31, 2016
Claim
Sep. 30, 2016
USD ($)
CustomerAccount
Claim
Dec. 09, 2014
Claim
Commitments And Contingencies [Line Items]              
Loss contingency, probable loss           $ 136,000,000  
Number of customer accounts | CustomerAccount           93,000  
Loss contingency, number of pending claims | Claim     52     73  
Loss contingency, number of dismissed claims | Claim     2        
Number of complaints filed | Complaint 4            
WVAWC [Member]              
Commitments And Contingencies [Line Items]              
Loss contingency, number of pending claims | Claim           53  
Loss contingency, number of additional claims | Claim     7        
WVAWC And AWWSC [Member]              
Commitments And Contingencies [Line Items]              
Loss contingency, number of pending claims | Claim           1  
Federal Action              
Commitments And Contingencies [Line Items]              
Loss contingency, number of pending claims | Claim             4
Loss contingency, number of dismissed claims | Claim       3      
Loss contingency, number of additional claims | Claim         3    
Binding Agreement              
Commitments And Contingencies [Line Items]              
Pre-tax amount of settlement           $ 126,000,000  
Insurance coverage           61,000,000  
Payment of settlement, after tax           $ 39,000,000  
Loss contingency preliminary terms of settlement           The preliminary terms of the Settlement intend to establish a two-tier settlement fund for the payment of claims, comprised of (i) a guaranteed fund of $76, of which $51 will be contributed by WVAWC, including insurance deductibles, and $25 would be contributed by one of the Company’s general liability insurance carriers, and (ii) a claims-based payment fund of up to $50, of which up to $14 would be contributed by WVAWC and $36 would be contributed by a number of the Company’s general liability insurance carriers. Separately, $25 would be contributed to the guaranteed fund through a settlement by another defendant to the Settlement  
Settlement guarantee fund contribution           $ 76,000,000  
Claims-based payment fund contribution           50,000,000  
Binding Agreement | Two Insurance Carrier              
Commitments And Contingencies [Line Items]              
Insurance coverage           50,000,000  
Binding Agreement | Insurance Carrier One              
Commitments And Contingencies [Line Items]              
Settlement guarantee fund contribution           25,000,000  
Binding Agreement | General Liability Insurance Carriers              
Commitments And Contingencies [Line Items]              
Claims-based payment fund contribution           36,000,000  
Binding Agreement | Other Defendants              
Commitments And Contingencies [Line Items]              
Settlement guarantee fund contribution           25,000,000  
Binding Agreement | WVAWC [Member]              
Commitments And Contingencies [Line Items]              
Payment of settlement           65,000,000  
Direct response costs           4,000,000  
Settlement guarantee fund contribution           51,000,000  
Claims-based payment fund contribution           14,000,000  
Maximum              
Commitments And Contingencies [Line Items]              
Loss contingency, possible loss           48,000,000  
Maximum | Missouri-American Water Company              
Commitments And Contingencies [Line Items]              
Loss contingency, possible loss           26,000,000  
Minimum | Missouri-American Water Company              
Commitments And Contingencies [Line Items]              
Loss contingency, possible loss           $ 0  
Wastewater Collection And Treatment System | Asset Purchase Agreement              
Commitments And Contingencies [Line Items]              
Amount of purchase price   $ 195,000,000          
Transfer of cash by acquiree   38,000,000          
Sewer system upgrades, estimated cost   $ 140,000,000