Retirement Benefits (Tables)

9 Months Ended

Sep. 30, 2017

Retirement Benefits [Abstract]

Pension Plans and Postretirement Plans

Components of the net periodic benefit costs for the three and nine months ended September 30, 2017 and 2016 are presented in the following tables.

Pension Plans	Southern Company			Alabama Power			Georgia Power			Gulf Power			Mississippi Power
	(in millions)
Three Months Ended September 30, 2017
Service cost	$	73		$	15		$	19		$	3		$	4
Interest cost	114			25			34			5			5
Expected return on plan assets	(224		)	(49		)	(71		)	(10		)	(9		)
Amortization:
Prior service costs	3			1			—			—			—
Net (gain)/loss	41			10			15			2			1
Net periodic pension cost (income)	$	7		$	2		$	(3	)	$	—		$	1
Nine Months Ended September 30, 2017
Service cost	$	220		$	47		$	56		$	10		$	11
Interest cost	341			73			103			15			15
Expected return on plan assets	(673		)	(147		)	(212		)	(29		)	(29		)
Amortization:
Prior service costs	9			2			2			—			1
Net (gain)/loss	122			31			43			5			5
Net periodic pension cost (income)	$	19		$	6		$	(8	)	$	1		$	3
Three Months Ended September 30, 2016
Service cost	$	68		$	14		$	17		$	3		$	3
Interest cost	110			23			34			5			4
Expected return on plan assets	(203		)	(46		)	(64		)	(9		)	(9		)
Amortization:
Prior service costs	3			1			1			—			1
Net (gain)/loss	45			10			14			2			2
Net periodic pension cost	$	23		$	2		$	2		$	1		$	1
Nine Months Ended September 30, 2016
Service cost	$	192		$	43		$	52		$	9		$	9
Interest cost	311			71			102			14			14
Expected return on plan assets	(577		)	(138		)	(193		)	(26		)	(26		)
Amortization:
Prior service costs	10			2			4			1			1
Net (gain)/loss	120			30			41			5			5
Net periodic pension cost	$	56		$	8		$	6		$	3		$	3

Pension Plans	Southern Company Gas
	(in millions)
Successor – Three Months Ended September 30, 2017
Service cost	$	6
Interest cost	10
Expected return on plan assets	(18		)
Amortization of net (gain)/loss	5
Net periodic pension cost	$	3
Successor – Nine Months Ended September 30, 2017
Service cost	$	17
Interest cost	30
Expected return on plan assets	(53		)
Amortization:
Prior service costs	(1		)
Net (gain)/loss	15
Net periodic pension cost	$	8
Successor – July 1, 2016 through September 30, 2016
Service cost	$	7
Interest cost	10
Expected return on plan assets	(17		)
Amortization of regulatory asset	6
Net periodic pension cost	$	6
Predecessor – January 1, 2016 through June 30, 2016
Service cost	$	13
Interest cost	21
Expected return on plan assets	(33		)
Amortization:
Prior service costs	(1		)
Net (gain)/loss	13
Net periodic pension cost	$	13

Postretirement Benefits	Southern Company			Alabama Power			Georgia Power			Gulf Power			Mississippi Power
	(in millions)
Three Months Ended September 30, 2017
Service cost	$	6		$	1		$	2		$	—		$	—
Interest cost	19			4			6			1			1
Expected return on plan assets	(16		)	(5		)	(6		)	—			—
Amortization:
Prior service costs	2			1			—			—			—
Net (gain)/loss	3			—			3			—			—
Net periodic postretirement benefit cost	$	14		$	1		$	5		$	1		$	1
Nine Months Ended September 30, 2017
Service cost	$	18		$	4		$	5		$	1		$	1
Interest cost	59			13			21			2			3
Expected return on plan assets	(49		)	(19		)	(18		)	(1		)	(1		)
Amortization:
Prior service costs	5			3			1			—			—
Net (gain)/loss	10			1			6			—			—
Net periodic postretirement benefit cost	$	43		$	2		$	15		$	2		$	3
Three Months Ended September 30, 2016
Service cost	$	6		$	1		$	2		$	—		$	—
Interest cost	20			5			7			1			—
Expected return on plan assets	(16		)	(6		)	(6		)	—			—
Amortization:
Prior service costs	1			1			—			—			—
Net (gain)/loss	5			—			3			—			1
Net periodic postretirement benefit cost	$	16		$	1		$	6		$	1		$	1
Nine Months Ended September 30, 2016
Service cost	$	17		$	4		$	5		$	1		$	1
Interest cost	55			14			22			2			2
Expected return on plan assets	(44		)	(19		)	(17		)	(1		)	(1		)
Amortization:
Prior service costs	4			3			1			—			—
Net (gain)/loss	12			1			7			—			1
Net periodic postretirement benefit cost	$	44		$	3		$	18		$	2		$	3

Postretirement Benefits	Southern Company Gas
	(in millions)
Successor – Three Months Ended September 30, 2017
Service cost	$	1
Interest cost	3
Expected return on plan assets	(2		)
Amortization:
Prior service costs	(1		)
Net (gain)/loss	1
Net periodic postretirement benefit cost	$	2
Successor – Nine Months Ended September 30, 2017
Service cost	$	2
Interest cost	8
Expected return on plan assets	(5		)
Amortization:
Prior service costs	(2		)
Net (gain)/loss	3
Net periodic postretirement benefit cost	$	6
Successor – July 1, 2016 through September 30, 2016
Service cost	$	1
Interest cost	2
Expected return on plan assets	(2		)
Amortization of regulatory asset	1
Net periodic postretirement benefit cost	$	2
Predecessor – January 1, 2016 through June 30, 2016
Service cost	$	1
Interest cost	5
Expected return on plan assets	(3		)
Amortization:
Prior service costs	(1		)
Net (gain)/loss	2
Net periodic postretirement benefit cost	$	4