Utility Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) - USD ($) $ in Millions		Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Regulatory Assets
Regulatory assets		$ 392.9	$ 395.1	$ 301.8
Regulatory Liabilities
Regulatory liabilities	[1]	58.7	55.6	63.2
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities	[1]	17.3	17.5	20.3
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities	[1]	23.8	22.3	28.1
State tax benefits—distribution system repairs
Regulatory Liabilities
Regulatory liabilities	[1]	15.6	15.1	13.7
Other
Regulatory Liabilities
Regulatory liabilities	[1]	2.0	0.7	1.1
Income taxes recoverable
Regulatory Assets
Regulatory assets		117.8	115.7	117.4
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets		179.4	183.1	138.3
Environmental costs
Regulatory Assets
Regulatory assets		61.4	59.4	17.6
Removal costs, net
Regulatory Assets
Regulatory assets		27.1	27.9	22.3
Other
Regulatory Assets
Regulatory assets		$ 7.2	$ 9.0	$ 6.2

[1] Regulatory liabilities are recorded in “other current liabilities” and “other noncurrent liabilities” on the Condensed Consolidated Balance Sheets.