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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary Of Nonvested Shares Related To Long-Term Incentive Plan
nonvested performance share unit and restricted stock unit activity for the year ended December 31, 2018:
 
Performance Share Units
 
Restricted Stock Units
 
Share
Units
 
Weighted-average Fair Value per Share Unit
 
Stock
Units
 
Weighted-average Fair Value per Stock Unit
Nonvested at January 1, 2018(a)
895,489

 
$
52.28

 
—

 
$
—

Granted
316,875

 
62.88

 
187,273

 
57.66

Forfeitures
(65,106
)
 
51.11

 
(5,463
)
 
58.99

Vested and undistributed(b)
(288,404
)
 
53.63

 
(26,557
)
 
59.02

Vested and distributed
(176,043
)
 
52.88

 
—

 
—

Nonvested at December 31, 2018(c)
682,811

 
$
56.58

 
155,253

 
$
57.38

(a)
Does not include 712,572 undistributed vested performance share units.
(b)
Vested and undistributed units are awards that vested due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three-year performance period.
(c)
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award
The following table presents the fair value of each share unit awarded under the 2014 Incentive Plan along with the significant assumptions used to calculate the fair value of each share unit for the years ended December 31, 2018, 2017, and 2016:
 
2018
2017
2016
Fair value of share units awarded
$62.88
$59.16
$44.13
Ameren’s closing common share price at December 31 of the prior year
$61.69
$52.46
$43.23
Three-year risk-free rate
1.98%
1.47%
1.31%
Volatility range for the peer group(a)
15% – 23%
15% – 21%
15% – 20%
(a)
Based on a historical period that is equal to the remaining term of the performance period as of the grant date.
The following table presents the stock-based compensation expense for the years ended December 31, 2018, 2017, and 2016:
 
2018
 
2017
 
2016
Ameren Missouri
$
4

 
$
4

 
$
4

Ameren Illinois
3

 
2

 
2

Other(a)
13

 
12

 
11

Ameren
20

 
18

 
17

Less income tax benefit
6

 
7

 
6

Stock-based compensation expense, net
$
14

 
$
11

 
$
11

(a)
Represents compensation expense of employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above.