Fair value measurement - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Measurements [Line Items]
Reversal of contingent consideration	$ 8,200,000	$ 12,300,000
Contingent consideration	33,433,000	20,313,000	51,196,000
Current portion of contingent consideration	11,276,000	4,131,000
Other liabilities	22,100,000
Minimum
Fair Value Measurements [Line Items]
Fair value of contingent consideration liability associated with future milestone	15,000,000
Maximum
Fair Value Measurements [Line Items]
Fair value of contingent consideration liability associated with future milestone	$ 83,000,000