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Retirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2017
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), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2017 and 2016:
  
Pension Benefits
 
Postretirement Benefits
 
 
Three Months
 
Nine Months
 
Three Months
 
Nine Months
 
  
2017
 
2016
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
 
Service cost
$
24

 
$
20

 
$
70

 
$
60

 
$
6

 
$
5

 
$
16

 
$
15

 
Interest cost
44

 
46

 
134

 
138

 
12

 
12

 
35

 
36

 
Expected return on plan assets
(65
)
 
(63
)
 
(196
)
 
(189
)
 
(19
)
 
(18
)
 
(56
)
 
(54
)
 
Amortization of:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Prior service benefit
(1
)
 
—

 
(1
)
 
—

 
(2
)
 
(1
)
 
(4
)
 
(3
)
 
Actuarial loss (gain)
14

 
8

 
41

 
24

 
(2
)
 
(3
)
 
(5
)
 
(8
)
 
Net periodic benefit cost (benefit)
$
16

 
$
11

 
$
48

 
$
33

 
$
(5
)
 
$
(5
)
 
$
(14
)
 
$
(14
)
 
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, 2017 and 2016:
  
Pension Benefits
 
Postretirement Benefits
 
 
Three Months
 
Nine Months
 
Three Months
 
Nine Months
 
  
2017
 
2016
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
 
Ameren Missouri(a)
$
6

 
$
6

 
$
18

 
$
19

 
$
(1
)
 
$
(1
)
 
$
(3
)
 
$
(3
)
 
Ameren Illinois
10

 
6

 
30

 
17

 
(3
)
 
(3
)
 
(10
)
 
(10
)
 
Other
—

 
(1
)
 
—

 
(3
)
 
(1
)
 
(1
)
 
(1
)
 
(1
)
 
Ameren(a)(b)
$
16

 
$
11

 
$
48

 
$
33

 
$
(5
)
 
$
(5
)
 
$
(14
)
 
$
(14
)
 

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