Share-based incentives - Additional Information (Detail) (Restricted Common Stock, USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted	$ 4.0	$ 2.2	$ 1.3
Share-based incentives, shares issued	124,000	82,000	63,000
Share-based incentives, average price of shares issued (usd per share)	$ 32.22	$ 26.13	$ 20.81