Utility Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	$ 235.9	$ 244.9	$ 322.3
Regulatory liabilities	33.1	37.3	56.7
Postretirement benefits
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	17.2	16.5	13.9
Environmental overcollections
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	1.9	2.6	3.0
Deferred fuel and power refunds
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	3.2	8.3	31.2
State tax benefits—distribution system repairs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	9.0	8.4	7.9
Other
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	1.8	1.5	0.7
Income taxes recoverable
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	106.8	106.1	104.2
Underfunded pension and postretirement plans
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	91.0	94.5	181.3
Environmental costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	14.5	17.1	16.8
Deferred fuel and power costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	4.3	8.3	2.1
Removal costs, net
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	14.4	13.3	11.9
Other
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	$ 4.9	$ 5.6	$ 6.0