Summary of Significant Accounting Policies - Narrative (Details)	3 Months Ended
Mar. 31, 2021
Accounting Policies [Abstract]
Maximum collection period	24 months

Represents the maximum collection period in which deferred net WRAM and MCBA revenues and associated costs will be recognized.

No definition available.

Name:	cwt_MaximumCollectionPeriodInWhichDeferredNetWRAMAndMCBARevenuesAndAssociatedCostsWillBeRecognized
Namespace Prefix:	cwt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration