Contingencies and Regulatory Matters - Gulf Power (Details) - GULF POWER CO $ in Millions			3 Months Ended
	Mar. 26, 2018 USD ($)	Jul. 01, 2017	Mar. 31, 2018 USD ($)	Mar. 31, 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
Retail regulatory equity ratio	0.535	0.525
Environmental Cost Recovery			$ 10.0		$ 11.0
Projected environmental expenditures			12.0	$ 13.0
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Fuel Cost Recovery			4.0		22.0
Purchased Power Capacity Recovery			4.0		2.0
Environmental Cost Recovery			$ 2.0		$ 2.0