Fair value measurements - Assets Measured at Fair Value on Nonrecurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financial Instruments and Fair Value [Line Items]
Impairment loss recorded	$ 1,000	$ 1,000	$ 800
Cost-basis investments
Financial Instruments and Fair Value [Line Items]
Impairment loss recorded	1,000	995
Level 3 | Cost-basis investments
Financial Instruments and Fair Value [Line Items]
Cost-basis investments, at fair value	$ 225	$ 1,000