Retirement Benefits (Tables)

3 Months Ended

Mar. 31, 2021

Retirement Benefits [Abstract]

Schedule of Pension Plans and Postretirement Plans

Components of the net periodic benefit costs for the three months ended March 31, 2021 and 2020 are presented in the following tables.

Three Months Ended March 31, 2021	Southern Company		Alabama Power		Georgia Power		Mississippi Power		Southern Power		Southern Company Gas
	(in millions)
Pension Plans
Service cost	$	109	$	26	$	28	$	4	$	2	$	9
Interest cost	87		20		26		4		1		6
Expected return on plan assets	(298)		(72)		(94)		(14)		(3)		(21)
Amortization:
Prior service costs	—		—		—		—		—		(1)
Regulatory asset	—		—		—		—		—		4
Net (gain)/loss	78		21		25		4		1		3
Net periodic pension cost (income)	$	(24)	$	(5)	$	(15)	$	(2)	$	1	$	—
Postretirement Benefits
Service cost	$	6	$	1	$	2	$	—	$	—	$	—
Interest cost	8		2		3		—		—		1
Expected return on plan assets	(19)		(7)		(7)		—		—		(2)
Amortization:
Regulatory asset	—		—		—		—		—		2
Net (gain)/loss	1		—		1		—		—		(1)
Net periodic postretirement benefit cost (income)	$	(4)	$	(4)	$	(1)	$	—	$	—	$	—

Three Months Ended March 31, 2020	Southern Company		Alabama Power		Georgia Power		Mississippi Power		Southern Power		Southern Company Gas
	(in millions)
Pension Plans
Service cost	$	94	$	22	$	24	$	4	$	2	$	8
Interest cost	108		25		33		5		1		8
Expected return on plan assets	(275)		(66)		(87)		(13)		(3)		(19)
Amortization:
Prior service costs	1		—		—		—		—		(1)
Regulatory asset	—		—		—		—		—		4
Net (gain)/loss	67		18		22		3		1		2
Net periodic pension cost (income)	$	(5)	$	(1)	$	(8)	$	(1)	$	1	$	2
Postretirement Benefits
Service cost	$	5	$	2	$	1	$	—	$	—	$	—
Interest cost	13		3		5		—		—		2
Expected return on plan assets	(18)		(7)		(7)		—		—		(2)
Amortization:
Regulatory asset	—		—		—		—		—		2
Net (gain)/loss	1		—		1		—		—		(1)
Net periodic postretirement benefit cost (income)	$	1	$	(2)	$	—	$	—	$	—	$	1