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	Dec. 31, 2014		Sep. 30, 2014		Dec. 31, 2013
Regulatory Assets And Liabilities
Regulatory assets	$ 270.6	[1]	$ 268.2	[1]	$ 233.9	[1]
Regulatory liabilities	32.9	[1]	32.5	[1]	36.6	[1]
Postretirement Benefits
Regulatory Assets And Liabilities
Regulatory liabilities	19.0	[1]	18.6	[1]	16.8	[1]
Environmental Overcollections
Regulatory Assets And Liabilities
Regulatory liabilities	0.2	[1]	0.3	[1]	2.3	[1]
Deferred Fuel and Power Refunds
Regulatory Assets And Liabilities
Regulatory liabilities	0	[1]	0.3	[1]	7.5	[1]
State Tax Benefits—Distribution System Repairs
Regulatory Assets And Liabilities
Regulatory liabilities	10.3	[1]	10.1	[1]	8.7	[1]
Other
Regulatory Assets And Liabilities
Regulatory liabilities	3.4	[1]	3.2	[1]	1.3	[1]
Income Taxes Recoverable
Regulatory Assets And Liabilities
Regulatory assets	111.1	[1]	110.7	[1]	106.4	[1]
Underfunded Pension and Postretirement Plans
Regulatory Assets And Liabilities
Regulatory assets	107.8	[1]	110.1	[1]	92.8	[1]
Environmental Costs
Regulatory Assets And Liabilities
Regulatory assets	14.7	[1]	14.6	[1]	14.9	[1]
Deferred Fuel and Power Costs
Regulatory Assets And Liabilities
Regulatory assets	16.7	[1]	11.8	[1]	0.4	[1]
Removal Costs, Net
Regulatory Assets And Liabilities
Regulatory assets	17.6	[1]	16.8	[1]	13.7	[1]
Other
Regulatory Assets And Liabilities
Regulatory assets	$ 2.7	[1]	$ 4.2	[1]	$ 5.7	[1]

[1] Noncurrent regulatory assets are recorded in other assets and regulatory liabilities are recorded in other current and other noncurrent liabilities in the Condensed Consolidated Balance Sheets.