Stockholders' equity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares
Stockholders Equity Note [Abstract]
Share repurchase program, authorized amount	$ 75,000,000
Common stock repurchased, shares | shares	293,991
Common stock repurchased, amount	$ 11,600,000
Common stock repurchased, price per share | $ / shares	$ 39.37
Share repurchase program, remaining authorized amount	$ 63,400,000