Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary Of Nonvested Shares Related To Long-Term Incentive Plan

nonvested performance share unit activity for the year ended December 31, 2017:

	Performance Share Units
	Share Units		Weighted-average Grant Date Fair Value per Share Unit
Nonvested at January 1, 2017(a)	780,545		$	47.54
Granted(b)	508,161		59.16
Forfeitures	(50,523	)	52.50
Undistributed vested units(c)	(342,694	)	51.65
Nonvested at December 31, 2017(a)	895,489		$	52.28

(a) Excludes 369,878 and 712,572 performance share units granted to retirement-eligible employees as of January 1, 2017 and December 31, 2017, respectively, as the undistributed performance share units are fully vested.

(b) Includes performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2017 under the 2014 Incentive Plan.

(c) Includes performance share units that vested due to attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]

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, 2017, 2016, and 2015:

	2017	2016	2015
Fair value of share units awarded	$59.16	$44.13	$52.88
Ameren’s closing common share price at December 31 of the prior year	$52.46	$43.23	$46.13
Three-year risk free rate	1.47%	1.31%	1.10%
Volatility range	15% - 21%	15% - 20%	12% - 18%

The following table presents the stock-based compensation expense for the years ended December 31, 2017, 2016, and 2015:

	2017		2016		2015
Ameren Missouri	$	4	$	4	$	5
Ameren Illinois	2		2		3
Other(a)	12		11		11
Ameren	18		17		19
Less income tax benefit	7		6		7
Stock-based compensation expense, net	$	11	$	11	$	12

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