Retirement Benefits (Tables)

3 Months Ended

Mar. 31, 2018

Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]

Components Of Net Periodic Benefit Cost

The following table presents the components of the net periodic benefit cost (income), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2018 and 2017:

	Pension Benefits						Postretirement Benefits
	Three Months						Three Months
	2018			2017			2018			2017
Service cost(a)	$	25		$	23		$	5		$	5
Non-service cost components:
Interest cost	42			45			11			12
Expected return on plan assets	(69		)	(66		)	(19		)	(19		)
Amortization of:
Prior service benefit	—			—			(1		)	(1		)
Actuarial loss (gain)	16			14			—			(2		)
Total non-service cost components(b)	(11		)	(7		)	(9		)	(10		)
Net periodic benefit cost (income)	$	14		$	16		$	(4	)	$	(5	)

(a) Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income.

(b) 2018 amounts and the non-capitalized portion of 2017’s non-service cost components, as discussed above, are reflected in “Other Income, Net” on Ameren’s statement of income.

Summary Of Benefit Plan Costs Incurred

Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of pension costs and the postretirement benefit costs (income) incurred for the three months ended March 31, 2018 and 2017:

	Pension Benefits				Postretirement Benefits
	Three Months				Three Months
	2018		2017		2018			2017
Ameren Missouri(a)	$	5	$	6	$	—		$	(1	)
Ameren Illinois	9		10		(4		)	(4		)
Ameren(a)	$	14	$	16	$	(4	)	$	(5	)

(a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.