Short-Term Debt - Additional Information (Details)		9 Months Ended	12 Months Ended
	Mar. 20, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Apr. 01, 2020 USD ($)
Short-term Debt [Line Items]
Total availability			$ 2,250,000,000	$ 2,250,000,000
Outstanding debt			(786,000,000)	(786,000,000)
Short-term debt			$ 1,388,000,000	1,388,000,000
Line of credit facility consolidated debt to consolidated capitalization ratio, required			0.70
Maturity in More Than Three Months
Short-term Debt [Line Items]
Proceeds from short-term borrowings with maturities greater than three months			$ 0
Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt			1,388,000,000	1,388,000,000
Revolving Credit Facility | Letter of Credit
Short-term Debt [Line Items]
Maximum borrowing capacity			150,000,000	150,000,000
Letters of credit outstanding, amount			$ 76,000,000	$ 76,000,000
American Water Capital Corp.
Short-term Debt [Line Items]
Maximum borrowing capacity					$ 500,000,000
Interest rate during period			1.16%	2.54%
American Water Capital Corp. | Term loan
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 750,000,000
Proceeds from lines of credit	500,000,000		$ 500,000,000
Additional maximum borrowing capacity	$ 250,000,000
Fair value of amount outstanding			500,000,000
American Water Capital Corp. | Term loan | London Interbank Offered Rate (LIBOR)
Short-term Debt [Line Items]
Debt, basis spread on variable rate	0.80%
American Water Capital Corp. | Revolving Credit Facility
Short-term Debt [Line Items]
Proceeds from lines of credit			$ 0
Proceeds from short-term debt		$ 650,000,000
Repayments of short-term debt		$ 650,000,000