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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Feb. 10, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2021    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-34028    
Entity Registrant Name AMERICAN WATER WORKS COMPANY, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 51-0063696    
Entity Address, Address Line One 1 Water Street    
Entity Address, City or Town Camden    
Entity Address, State or Province NJ    
Entity Address, Postal Zip Code 08102-1658    
City Area Code 856    
Local Phone Number 955-4001    
Title of 12(b) Security Common stock, par value $0.01 per share    
Trading Symbol AWK    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 24,667.4
Entity Common Stock, Shares Outstanding (in shares)   181,724,991  
Documents Incorporated by Reference
DOCUMENTS INCORPORATED BY REFERENCE
Portions of the American Water Works Company, Inc. definitive proxy statement for the 2022 Annual Meeting of Shareholders to be filed with the Securities and Exchange Commission within 120 days after December 31, 2021 are incorporated by reference into Part III of this report.
    
Entity Central Index Key 0001410636    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Amendment Flag false