Summary of significant accounting policies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2016
Maximum
Significant Accounting Policies [Line Items]
Finite-Lived Intangible Assets, Useful Life	10 years
Minimum
Significant Accounting Policies [Line Items]
Finite-Lived Intangible Assets, Useful Life	5 years

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration