Income taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income tax expense at federal statutory rate, rate	35.00%	35.00%
Unrecognized income tax benefits	$ 21.0		$ 21.0		$ 21.6
Recognized additional liabilities			1.2	0.9
Reversed prior year liabilities			$ 1.7