Income taxes - Changes in Uncertain Tax Positions (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unrecognized tax benefits:
Amount beginning of year	$ 23.0	$ 37.4	$ 31.8
Net increase:
Tax positions taken in prior periods	1.1	0.5	0.3
Tax positions taken in current period	11.0	6.0	5.4
Settlements with taxing authorities - cash paid	(0.1)
Lapse due to applicable statute of Limitations	(1.8)	(20.6)	(0.5)
Changes in currency exchange rates	0.2	(0.3)	0.4
Amount at end of year	$ 33.4	$ 23.0	$ 37.4