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Fair Value Measurements Text Tag (Details) | 12 Months Ended
--- | ---
Mar. 31, 2018 USD ($)
Fair Value Disclosures [Abstract] |
Percent of Total Assets Level 3 | 0.10%
Fair Value, Measurement with Unobservable Inputs, Recurring Basis, Asset, Transfers, Net | $ 0

X
- Definition Percent of Total Assets Level 3 + References No definition available. + Details
Name: | carv_PercentOfTotalAssetsLevel3
Namespace Prefix: | carv_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy. + References No definition available. + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration