Consolidated Statements of Comprehensive Income (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Statement of Other Comprehensive Income [Abstract]
Tax on (loss) gain on derivative instruments	$ (7.2)	$ 29.3	$ 5.0
Tax on reclassifications on derivative instruments	(10.3)	(14.6)	1.5
Tax on foreign currency translation	(6.6)	2.8	4.5
Tax on foreign currency gain and losses on long-term intra-company transactions	(1.7)	0.7	0.4
Tax on benefit plans	(3.8)	6.0	(0.1)
Tax on reclassification of benefit plans and prior service costs	(0.8)	(0.5)	(0.4)
Reclassification of pension plans actuarial losses and prior service costs to regulatory assets, tax	$ 0	$ 0	$ 0