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Reconciliation of Income (Loss) from Continuing Operations, (Loss) from Discontinued Operations and Net Income (Loss) and Weighted Average Common Shares Outstanding for Calculating Diluted Earnings (Loss) Per Share (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Earnings Per Share Diluted [Line Items]                      
Total income from continuing operations available to common shareholders, basic                 $ 429,721awk_IncomeLossFromContinuingOperationsAvailableToCommonStockholdersBasic $ 370,700awk_IncomeLossFromContinuingOperationsAvailableToCommonStockholdersBasic $ 373,458awk_IncomeLossFromContinuingOperationsAvailableToCommonStockholdersBasic
Loss from discontinued operations, net of tax                 (6,733)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (1,580)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (15,532)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax
Total income available to common shareholders, basic                 422,988us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic 369,120us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic 357,926us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Undistributed earnings for participating securities                 57us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesDiluted 84us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesDiluted 58us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesDiluted
Total income from continuing operations available to common shareholders, diluted                 429,778awk_IncomeLossFromContinuingOperationsAvailableToCommonStockholdersDiluted 370,784awk_IncomeLossFromContinuingOperationsAvailableToCommonStockholdersDiluted 373,516awk_IncomeLossFromContinuingOperationsAvailableToCommonStockholdersDiluted
Total income available to common shareholders, diluted                 $ 423,045us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted $ 369,204us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted $ 357,984us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted
Weighted-average common shares outstanding, basic                 178,888us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 177,814us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 176,445us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Weighted-average common shares outstanding, diluted                 179,806us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 179,056us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 177,671us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Diluted earnings per share:                      
Income from continuing operations $ 0.52us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.87us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.61us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.39us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.34us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.84us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.57us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.33us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 2.39us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare [1],[2] $ 2.07us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare [1],[2] $ 2.10us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare [1],[2]
Loss from discontinued operations, net of tax                 $ (0.04)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare [1],[2] $ (0.01)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare [1],[2] $ (0.09)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare [1],[2]
Net income $ 0.52us-gaap_EarningsPerShareDiluted $ 0.85us-gaap_EarningsPerShareDiluted $ 0.61us-gaap_EarningsPerShareDiluted $ 0.38us-gaap_EarningsPerShareDiluted $ 0.33us-gaap_EarningsPerShareDiluted $ 0.84us-gaap_EarningsPerShareDiluted $ 0.57us-gaap_EarningsPerShareDiluted $ 0.32us-gaap_EarningsPerShareDiluted $ 2.35us-gaap_EarningsPerShareDiluted [1],[2] $ 2.06us-gaap_EarningsPerShareDiluted [1],[2] $ 2.01us-gaap_EarningsPerShareDiluted [1],[2]
Restricted Stock Units                      
Earnings Per Share Diluted [Line Items]                      
Restricted stock units                 454awk_IncrementalCommonSharesAttributableToRestrictedStockUnits
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 510awk_IncrementalCommonSharesAttributableToRestrictedStockUnits
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 618awk_IncrementalCommonSharesAttributableToRestrictedStockUnits
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
Employee Stock Purchase Plan                      
Earnings Per Share Diluted [Line Items]                      
Employee stock purchase plan                 1awk_IncrementalCommonSharesAttributableToEmployeeStockPurchasePlan
/ us-gaap_AwardTypeAxis
= awk_EmployeeStockPurchasePlanMember		 2awk_IncrementalCommonSharesAttributableToEmployeeStockPurchasePlan
/ us-gaap_AwardTypeAxis
= awk_EmployeeStockPurchasePlanMember		 1awk_IncrementalCommonSharesAttributableToEmployeeStockPurchasePlan
/ us-gaap_AwardTypeAxis
= awk_EmployeeStockPurchasePlanMember
Stock Options                      
Earnings Per Share Diluted [Line Items]                      
Stock options                 463awk_IncrementalCommonSharesAttributableToStockOptions
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember		 730awk_IncrementalCommonSharesAttributableToStockOptions
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember		 607awk_IncrementalCommonSharesAttributableToStockOptions
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember
[1] Amounts may not sum due to rounding
[2] Earnings per share amounts are computed independently for income from continuing operations, loss from discontinued operations and net income. As a result, the sum of per-share amounts from continuing operations and discontinued operations may not equal the total per-share amount for net income.