Retirement Benefits - Defined Benefit Plan Related to Gulf Power (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 12,763	$ 13,808	$ 12,385
Plan assets	11,611	12,992	11,583
Accrued liability	(1,152)	(816)
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	1,865	2,339	2,297
Plan assets	928	1,053	$ 944
Accrued liability	(937)	$ (1,286)
Gulf Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	526
Plan assets	492
Accrued liability	(34)
Gulf Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	69
Plan assets	17
Accrued liability	$ (52)