Contingencies and Regulatory Matters - Cost Recovery Clause (Details) (Gulf Power [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Loss Contingencies [Line Items]
Fuel Cost Recovery – Under	$ 41.3
Under Recovered Regulatory Clause Revenues [Member]
Loss Contingencies [Line Items]
Fuel Cost Recovery – Under	41.3	21.0
Purchased Power Capacity Recovery – Under	0	2.8
Environmental Cost Recovery – Under	6.3	14.4
Energy Conservation Cost Recovery – Under	2.6	7.0
Other regulatory liabilities current [Member]
Loss Contingencies [Line Items]
Purchased Power Capacity Recovery – Over	$ 6.8	$ 0