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

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

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

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

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

Represents the period over which customer advances will be refunded.

No definition available.

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

- References

No definition available.

Name:	cwt_RegulatoryAssetsAndLiabilitiesLineItems
Namespace Prefix:	cwt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Name:	us-gaap_AmortizationOfDebtDiscountPremium
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of liability for consideration received or receivable from customer which is not included in transaction price, when consideration is expected to be refunded to customer, classified as noncurrent.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 55
-Paragraph 27
-URI http://asc.fasb.org/extlink&oid=108792157&loc=SL49130611-203046-203046

Name:	us-gaap_ContractWithCustomerRefundLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

Name:	us-gaap_CustomerAdvancesOrDepositsNoncurrentAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	us-gaap_InterestExpenseDebtAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RegulatoryAssetsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

Name:	us-gaap_RegulatoryAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

Name:	us-gaap_RegulatoryLiabilityCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_RegulatoryLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 55
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82849221&loc=d3e44360-110383

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RemainingRecoveryPeriodOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_RegulatoryLiabilityAxis=us-gaap_DeferredIncomeTaxChargesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_HealthCareBalancingAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_ConservationProgramLiabilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_NetWRAMandMCBALongtermPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_PensionBalancingAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_TaxAccountingMemorandumAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_CostofCapitalMemorandumAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_A123TrichloropropaneSettlementProceedsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryLiabilityAxis=cwt_RegulatoryLiabilityOtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_PensionAndPostRetirementBenefitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_PropertyRelatedTemporaryDifferencesTaxBenefitsFlowedThroughToCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_OtherAccruedBenefitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_NetWRAMandMCBALongTermAccountsReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_AssetRetirementObligationCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_InterimRatesLongTermAccountsReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_TankCoatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_HealthCareBalancingAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_PensionBalancingAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_PostretirementBenefitCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RegulatoryAssetAxis=cwt_OtherRegulatoryAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: