Contingencies and Regulatory Matters - Gulf Power (Details) - GULF POWER CO $ in Millions						9 Months Ended	12 Months Ended
	Nov. 05, 2018 USD ($)	Oct. 30, 2018 USD ($)	Oct. 10, 2018 USD ($)	Mar. 26, 2018 USD ($) intervenor	Mar. 25, 2018	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Property damage reserve						$ 48.0
Property damage reserve replenishment						40.0
Number of intervenors | intervenor				3
Tax reform settlement agreement, annual reduction of revenue from base rates				$ 18.2
Tax reform settlement agreement, annual reduction of revenue from environmental cost recovery rates				15.6
Tax reform settlement agreement, refund				$ 69.4
Customer refundable fees, refund payments						53.0
Retail regulatory equity ratio				0.535	0.525
Environmental Cost Recovery						21.0	$ 11.0
Projected environmental expenditures						8.0	13.0
Property Basis Differences
Loss Contingencies [Line Items]
Deferred tax asset, regulatory liability						7.0
Subsequent Event
Loss Contingencies [Line Items]
Recovery from customers, period following the filing of cost recovery petition			60 days
Tax reform settlement agreement, annual reduction of revenue from base rates		$ 9.6
Decrease in annual revenues	$ 38.0
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Fuel Cost Recovery						0.0	22.0
Purchased Power Capacity Recovery						0.0	2.0
Environmental Cost Recovery						0.0	2.0
Other regulatory liabilities, current
Loss Contingencies [Line Items]
Fuel Cost Recovery						23.0	0.0
Purchased Power Capacity Recovery						4.0	0.0
Environmental Cost Recovery						13.0	0.0
Energy Conservation Cost Recovery						$ 2.0	$ 0.0
Loss from Catastrophes | Subsequent Event | Minimum
Loss Contingencies [Line Items]
Estimated restoration costs			$ 350.0
Loss from Catastrophes | Subsequent Event | Maximum
Loss Contingencies [Line Items]
Estimated restoration costs			$ 400.0