Retirement Benefits (Tables)

9 Months Ended

Sep. 30, 2015

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 (benefit) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2015 and 2014:

	Pension Benefits												Postretirement Benefits
	Three Months						Nine Months						Three Months						Nine Months
	2015			2014			2015			2014			2015			2014			2015			2014
Service cost	$	23		$	20		$	69		$	60		$	6		$	5		$	17		$	14
Interest cost	43			46			130			137			12			12			36			37
Expected return on plan assets	(62		)	(58		)	(186		)	(172		)	(17		)	(16		)	(51		)	(48		)
Amortization of:
Prior service benefit	—			(1		)	—			(1		)	(1		)	(2		)	(3		)	(4		)
Actuarial loss (gain)	19			13			56			37			1			(2		)	4			(5		)
Settlement loss	—			—			1			—			—			—			—			—
Net periodic benefit cost (benefit)	$	23		$	20		$	70		$	61		$	1		$	(3	)	$	3		$	(6	)

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 pension costs and the postretirement benefit costs (benefit) incurred for the three and nine months ended September 30, 2015 and 2014:

	Pension Benefits								Postretirement Benefits
	Three Months				Nine Months				Three Months						Nine Months
	2015		2014		2015		2014		2015			2014			2015			2014
Ameren Missouri(a)	$	14	$	13	$	42	$	38	$	2		$	—		$	6		$	2
Ameren Illinois	9		7		28		22		—			(3		)	(2		)	(7		)
Other	—		—		—		1		(1		)	—			(1		)	(1		)
Ameren(a)(b)	$	23	$	20	$	70	$	61	$	1		$	(3	)	$	3		$	(6	)

(a) The amounts above do 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 built into rates.

(b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.