Fair value measurements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Impairment loss recorded	$ 600	$ 1,000

For investments in debt and equity securities accounted for at cost, the excess of the carrying amount over net sale proceeds of investments disposed of during the period and any losses recognized thereon for impairments of other than a temporary nature.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9(a))
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Name:	us-gaap_CostMethodInvestmentsRealizedLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration