Employee benefit plans - Schedule of Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Pension Plans [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial gain (loss) arising during the year	$ (66.7)	$ (30.1)	$ (11.4)
Amortization of unrecognized prior service cost (credit)	1.1	1.2	0.9
Amortization of unrecognized prior service cost net transition obligations	0.4	0.5	0.5
Amortization of unrecognized net actuarial gain/loss	9.7	8.2	7.0
Plan amendments			(3.8)
Total	(55.5)	(20.2)	(6.8)
OPEB [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial gain (loss) arising during the year	(0.4)	(0.9)	(2.3)
Amortization of unrecognized prior service cost (credit)	(1.8)	(2.3)	(0.7)
Plan amendments			9.5
Total	$ (2.2)	$ (3.2)	$ 6.5