Stock option activity (Detail) - Stock Options $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award
Shares Exercisable | shares	744
Weighted Average Exercise Price of Exercisable Shares | $ / shares	$ 23.00
Weighted Average Exercise Price of Shares Outstanding at Beginning of Period | $ / shares	20.23
Weighted Average Exercise Price of Shares Exercised | $ / shares	11.37
Weighted Average Exercise Price of Shares Outstanding at End of Period | $ / shares	$ 23.02
Weighted Average Remaining Contractual Term of Outstanding Options	3 years 2 months 12 days
Weighted Average Remaining Contractual Term of Exercisable Option	3 years 1 month 6 days
Options Outstanding, Intrinsic Value | $	$ 7.8
Options, Exercisable, Intrinsic Value | $	$ 7.7
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding
Shares Outstanding at Beginning of Period | shares	999
Shares Exercised | shares	(240)
Shares Outstanding at End of Period | shares	759