Share-based compensation - Unvested Balance of Options (Detail) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of units
Unvested, Beginning balance (in shares)	47,500	68,750
Vested (in shares)	(47,500)	(21,250)
Unvested, Ending balance (in shares)	0	47,500	68,750
Weighted average exercise price per share
Unvested, Beginning balance (in dollars per share)	$ 8.96	$ 8.91
Vested (in dollars per share)	8.96	8.80
Unvested, Ending balance (in dollars per share)	$ 0.00	$ 8.96	$ 8.91
Weighted average remaining contractual term
Unvested	0 years	7 years 10 months 24 days	8 years 10 months 24 days

Share-based compensation arrangement by share based payment award options non-vested weighted average remaining contractual term.

No definition available.

Name:	cogt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedWeightedAverageRemainingContractualTerm
Namespace Prefix:	cogt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Sharebased compensation arrangement by sharebased payment award options nonvested weighted average exercise price per share.

No definition available.

Name:	cogt_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageExercisePricePerShare
Namespace Prefix:	cogt_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

Sharebased compensation arrangement by sharebased payment award options nonvested weighted average exercise price per share.

No definition available.

Name:	cogt_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageExercisePricePerShareAbstract
Namespace Prefix:	cogt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Sharebased compensation arrangement by sharebased payment award options vested weighted average exercise price per share.

No definition available.

Name:	cogt_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePricePerShare
Namespace Prefix:	cogt_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

No definition available.

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

Number of non-vested options outstanding.

No definition available.

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

Number of options vested.

No definition available.

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration