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Long-Term Debt - Schedule of Maturities of Long-Term Debt (Details) $ in Thousands	Sep. 30, 2025 USD ($)
Debt Disclosure [Abstract]
2025	$ 0
2026	417
2027	5,000
2028	4,583
Total	$ 10,000

X

- Definition

Long Term Debt, Principal

+ References

No definition available.

+ Details

Name:	cstl_LongTermDebtPrincipal
Namespace Prefix:	cstl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Long-Term Debt, Principal, Maturity, Remainder of Fiscal Year

+ References

No definition available.

+ Details

Name:	cstl_LongTermDebtPrincipalMaturityRemainderOfFiscalYear
Namespace Prefix:	cstl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Long-Term Debt, Principal, Maturity, Year One

+ References

No definition available.

+ Details

Name:	cstl_LongTermDebtPrincipalMaturityYearOne
Namespace Prefix:	cstl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Long-Term Debt, Principal, Maturity, Year Three

+ References

No definition available.

+ Details

Name:	cstl_LongTermDebtPrincipalMaturityYearThree
Namespace Prefix:	cstl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Long-Term Debt, Principal, Maturity, Year Two

+ References

No definition available.

+ Details

Name:	cstl_LongTermDebtPrincipalMaturityYearTwo
Namespace Prefix:	cstl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

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