Electric Rate and Regulatory Matters (Details) $ in Millions						2 Months Ended	8 Months Ended	12 Months Ended
	Feb. 20, 2018 USD ($) MW	Jun. 13, 2017	May 18, 2017 USD ($)	Mar. 07, 2017	Feb. 23, 2017 USD ($)	Apr. 30, 2017 USD ($)	Oct. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 30, 2017 USD ($) project MW	Sep. 28, 2016	Jun. 30, 2016	Jan. 06, 2015	Nov. 12, 2013
Public Utilities, General Disclosures [Line Items]
Number of solar projects filed for authority to recover costs | project											3
Additional solar generation resulting from project | MW											4
Additional battery storage resources resulting from project | MW											1
Estimated cost of equipment to comply with MATS								$ 12.8
Estimated cost of equipment required by the NOV								4.7
SIGECO Electric Demand Side Managements (DSM) Program Filing
Life of the plan		9 years
Electric utility revenue								569.6	$ 605.8	$ 601.6
FERC Return On Equity Complaint [Abstract]
Current return on equity used in MISO transmission owners rates															12.38%
Reduced return on equity percentage sought by third party through joint complaint															9.15%
FERC authorized base ROE percentage for first refund period												10.32%
FERC authorized base ROE percentage for second refund period													9.70%
FERC authorized incentive ROE percentage for first refund period														0.50%
Gross investment in qualifying projects								157.7
Net plant balance for qualifying projects								133.5
Electric Modernization Program - SB 560
Public Utilities, General Disclosures [Line Items]
Capital expenditures under plan					$ 500.0
Length of project plan required for recovery under new legislation					7 years
Project spend life to date			$ 446.0
Investment under plan						$ 7.1
Requested recovery in rates of investments							$ 31.0
Regulatory assets								4.3
Renewable Generation - SB 29
Public Utilities, General Disclosures [Line Items]
Estimated cost of project											$ 16.0
SIGECO Electric Environmental Compliance Filing
Public Utilities, General Disclosures [Line Items]
Amount of capital investments to date in equipment for mercury control								30.0
Amount of capital investments to date in equipment to control sulfur trioxide emissions								40.0
Electric Demand Side Management Filing
SIGECO Electric Demand Side Managements (DSM) Program Filing
Maximum period for recovery under energy efficiency program				4 years
Electric utility revenue								$ 11.6	$ 11.1	$ 10.1
Electric Generation and Compliance [Member] | Subsequent Event
FERC Return On Equity Complaint [Abstract]
Gas combined cycle generating facility cost	$ 900.0
Requested recovery of environmental investments	$ 90.0
Universal solar energy power | MW	50
Electric Generation and Compliance [Member] | Subsequent Event | Minimum
FERC Return On Equity Complaint [Abstract]
Gas combined cycle generating facility | MW	800
Electric Generation and Compliance [Member] | Subsequent Event | Maximum
FERC Return On Equity Complaint [Abstract]
Gas combined cycle generating facility | MW	900