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Fair Value Measurements Fair Value Measurements Text Tag (Details) | 6 Months Ended
--- | ---
Sep. 30, 2017
Fair Value Disclosures [Abstract] |
Percent of Total Assets Level 3 | 0.10%
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