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Fair Value Measurements Fair Value Measurements Text Tag (Details)	6 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Percent of Total Assets Level 3	0.11%
Transfers between level 1 and level 2	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

- Definition

Transfers between level 1 and level 2

+ References

No definition available.

+ Details

Name:	carv_Transfersbetweenlevel1andlevel2
Namespace Prefix:	carv_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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