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Fair Value Measurements Text Tag (Details) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Percent of Total Assets Level 3	0.11%	0.09%
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