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Pension Plans And Other Postretirement Benefits (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]  
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year $ 6,878us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear
Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Cash contributions made by the Company $ 6,878us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= wtr_DefinedBenefitPensionPlansMember