Employee benefit plans - Defined Benefit Plans and Other Postretirement Benefits (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined benefit plans [Member]
Defined benefit plans
Service cost	$ 2.5	$ 2.8	$ 7.7	$ 8.3
Interest cost	6.2	6.9	19.1	20.5
Expected return on plan assets	(5.5)	(5.8)	(16.5)	(17.2)
Amortization of prior service cost	0.4	0.3	1.2	1.1
Amortization net transition obligations	0.1	0.2	0.3	0.4
Recognized actuarial losses	2.3	1.9	6.9	5.6
Total	6.0	6.3	18.7	18.7
Other postretirement benefits [Member]
Defined benefit plans
Service cost	0.1	0.1	0.2	0.2
Interest cost	0.1	0.2	0.6	0.7
Amortization of prior service cost	(0.5)	(0.6)	(1.4)	(1.7)
Recognized actuarial losses	0.1		0.1
Total	$ (0.2)	$ (0.3)	$ (0.5)	$ (0.8)