Employee benefit plans - Components of Net Periodic Defined Benefit Pension Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.1	$ 0.1
Interest cost	0.2	0.2
Amortization of unrecognized: Prior service cost	(0.5)	(0.5)
Total	(0.2)	(0.2)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3.3	2.6
Interest cost	6.1	6.4
Expected return on plan assets	(6.1)	(5.5)
Amortization of unrecognized: Prior service cost	0.4	0.4
Amortization of unrecognized: Net transition obligations	0.1	0.1
Actuarial losses	3.4	2.3
Total	$ 7.2	$ 6.3