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	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2013
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	$ 242.6	$ 244.9	$ 320.9
Regulatory liabilities	30.3	37.3	40.0
Postretirement benefits
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	17.5	16.5	14.2
Environmental overcollections
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	1.6	2.6	2.9
Deferred fuel and power refunds
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	0	8.3	14.2
State tax benefits—distribution system repairs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	9.3	8.4	8.0
Other
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory liabilities	1.9	1.5	0.7
Income taxes recoverable
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	107.2	106.1	104.7
Underfunded pension and postretirement plans
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	89.2	94.5	177.8
Environmental costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	14.6	17.1	16.6
Deferred fuel and power costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	9.4	8.3	4.1
Removal costs, net
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	15.6	13.3	12.1
Other
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	$ 6.6	$ 5.6	$ 5.6