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Fair Value Measurements - Additional Information (Detail)	Dec. 31, 2017 USD ($)
Fair Value Disclosures [Abstract]
Direct investments	$ 0

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- Definition

Direct Investments

+ References

No definition available.

+ Details

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Namespace Prefix:	ck0001097396_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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