Regulatory Matters - Gulf Power Regulatory Matters (Details) - Gulf Power - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 31, 2017	Mar. 31, 2017	Dec. 31, 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 54.0
Rate case settlement annual increase base rates	62.0
Purchased power capacity cost recovery clause credit	$ 8.0
Tax reform settlement agreement, annual reduction of revenue from base rates			$ 18.0
Tax reform settlement agreement, annual reduction of revenue from environmental cost recovery rates			16.0
Tax reform settlement agreement, refund			$ 69.0
Plant Scherer Unit 3
Schedule of Regulatory Assets and Liabilities [Line Items]
Loss on Plant Scherer Unit 3		$ 32.5