Utility Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) - USD ($) $ in Millions		Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014
Regulatory Assets And Liabilities
Regulatory assets	[1]	$ 254.2	$ 268.2	$ 242.6
Regulatory liabilities	[1]	77.5	32.5	30.3
Postretirement Benefits
Regulatory Assets And Liabilities
Regulatory liabilities	[1]	19.6	18.6	17.5
Environmental Overcollections
Regulatory Assets And Liabilities
Regulatory liabilities	[1]	0.0	0.3	1.6
Deferred Fuel and Power Refunds
Regulatory Assets And Liabilities
Regulatory liabilities	[1]	45.6	0.3	0.0
State Tax Benefits—Distribution System Repairs
Regulatory Assets And Liabilities
Regulatory liabilities	[1]	10.9	10.1	9.3
Other
Regulatory Assets And Liabilities
Regulatory liabilities	[1]	1.4	3.2	1.9
Income Taxes Recoverable
Regulatory Assets And Liabilities
Regulatory assets	[1]	111.8	110.7	107.2
Underfunded Pension and Postretirement Plans
Regulatory Assets And Liabilities
Regulatory assets	[1]	103.2	110.1	89.2
Environmental Costs
Regulatory Assets And Liabilities
Regulatory assets	[1]	14.5	14.6	14.6
Deferred Fuel and Power Costs
Regulatory Assets And Liabilities
Regulatory assets	[1]	0.0	11.8	9.4
Removal Costs, Net
Regulatory Assets And Liabilities
Regulatory assets	[1]	19.6	16.8	15.6
Other
Regulatory Assets And Liabilities
Regulatory assets	[1]	$ 5.1	$ 4.2	$ 6.6

[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.