Contingencies and Regulatory Matters - Gulf Power (Details) - GULF POWER CO $ in Millions			3 Months Ended	6 Months Ended
	Mar. 26, 2018 USD ($)	Mar. 25, 2018	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
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			$ 28.0
Retail regulatory equity ratio	0.535	0.525
Environmental Cost Recovery			10.0	$ 10.0		$ 11.0
Projected environmental expenditures				11.0	$ 13.0
Property Basis Differences
Loss Contingencies [Line Items]
Deferred tax asset, regulatory liability			5.0	5.0
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Fuel Cost Recovery			0.0	0.0		22.0
Purchased Power Capacity Recovery			2.0	2.0		2.0
Environmental Cost Recovery			1.0	1.0		2.0
Other regulatory liabilities, current
Loss Contingencies [Line Items]
Fuel Cost Recovery			5.0	5.0		0.0
Energy Conservation Cost Recovery			$ 1.0	$ 1.0		$ 0.0