Share-based compensation - Unvested Balance of Options (Detail) - $ / shares	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Number of options
Unvested, beginning balance	47,500
Vested	(47,500)
Unvested, ending balance	0	47,500
Weighted average exercise price per share
Unvested, beginning balance	$ 8.96
Vested	8.96
Unvested, ending balance	$ 0.00	$ 8.96
Weighted average remaining contractual term
Unvested		7 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.

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

No definition available.

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

No definition available.

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

No definition available.

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No definition available.

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Number of non-vested options outstanding.

No definition available.

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

No definition available.

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Number of options vested.

No definition available.

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