Fair value measurements - Assets Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment loss recorded	$ 600	$ 1,000
Level 3 | Cost-basis investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost-basis investments, at fair value	$ 3,127	$ 3,938