RETIREMENT BENEFITS - Defined Benefit Plan Related to Gulf Power (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 14,788	$ 12,763	$ 13,808
Plan assets	14,057	11,611	12,992
Accrued liability	(731)	(1,152)
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	1,985	1,865	2,339
Plan assets	1,061	928	$ 1,053
Accrued liability	(924)	$ (937)
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)