Pension plans and other postretirement benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Pension and Other Post-retirement Benefits	$ (20.3)
Pension curtailment	0	37.4
Comprehensive income	112.3	128.3
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension curtailments, after-tax	23.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	3.7	7.0
Interest cost	13.8	12.3
Expected return on plan assets	(17.5)	(17.0)
Amortization of prior service cost	0	(0.4)
Amortization of unrecognized net loss	4.7	7.4
Settlement charge	0.1	0
Curtailment gain	0	(37.4)
Net periodic benefit cost/(income)	4.8	(28.1)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.5	0.2
Interest cost	0.2	0.3
Amortization of prior service cost	0	(0.9)
Amortization of unrecognized net loss	0	(0.5)
Net periodic benefit cost/(income)	0.7	(0.9)
Scenario, Previously Reported [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Comprehensive income		151.8
Scenario, Actual [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Comprehensive income		$ 128.3