Utility Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	$ 330.6	$ 338.4	$ 301.5
Regulatory Liabilities	26.8	28.3	28.4
Postretirement benefits [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liabilities	13.5	13.1	11.8
Environmental overcollections [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liabilities	3.1	2.9	4.7
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liabilities	1.9	4.4	5.0
State tax benefits - distribution system repairs [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liabilities	7.7	7.4	6.5
Other [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liabilities	0.6	0.5	0.4
Income taxes recoverable [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	103.7	103.2	98.7
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	184.8	188.2	148.7
Environmental costs [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	17.1	16.8	19.4
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	7.8	11.6	14.8
Removal costs, net [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	11.5	12.7	11.9
Other [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	$ 5.7	$ 5.9	$ 8.0