SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Advances for Construction (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Accounting Policies [Abstract]
2021	$ 8,935
2022	8,841
2023	8,791
2024	8,689
2025	8,434
Thereafter	151,935
Total refunds	$ 195,625

Customer Advances For Construction Refunds Due

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Customer Advances For Construction Refunds Due Thereafter

No definition available.

Name:	cwt_CustomerAdvancesForConstructionRefundsDueThereafter
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Represents the amount of advances for construction expected to be refunded to customers during year five of the five succeeding fiscal years.

No definition available.

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Represents the amount of advances for construction expected to be refunded to customers during year four of the five succeeding fiscal years.

No definition available.

Name:	cwt_CustomerAdvancesForConstructionRefundsDueYearFour
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Represents the amount of advances for construction expected to be refunded to customers during year one of the five succeeding fiscal years.

No definition available.

Name:	cwt_CustomerAdvancesForConstructionRefundsDueYearOne
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Represents the amount of advances for construction expected to be refunded to customers during year three of the five succeeding fiscal years.

No definition available.

Name:	cwt_CustomerAdvancesForConstructionRefundsDueYearThree
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Data Type:	xbrli:monetaryItemType
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Represents the amount of advances for construction expected to be refunded to customers during year two of the five succeeding fiscal years.

No definition available.

Name:	cwt_CustomerAdvancesForConstructionRefundsDueYearTwo
Namespace Prefix:	cwt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

No definition available.

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