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Summary of Significant Accounting Policies - Narrative (Details) | 9 Months Ended
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Sep. 30, 2021
Accounting Policies [Abstract] |
Maximum collection period | 24 months

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- Definition Represents the maximum collection period in which deferred net WRAM and MCBA revenues and associated costs will be recognized. + References No definition available. + Details
Name: | cwt_MaximumCollectionPeriodInWhichDeferredNetWRAMAndMCBARevenuesAndAssociatedCostsWillBeRecognized
Namespace Prefix: | cwt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration