Stock-Based Compensation (Summary of Expense) (Details) - Performance Share Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 20	$ 18	$ 17
Employee service share-based compensation, tax benefit from compensation expense	6	7	6
Share-based Compensation Expense, Net of Tax	$ 14	$ 11	$ 11
Performance period	3 years
Weighted-average Fair Value per Unit, Granted (in dollars per share)	$ 62.88	$ 59.16	$ 44.13
Closing common share price (in dollars per share)		$ 61.69	$ 52.46	$ 43.23
Three-year risk-free rate	1.98%	1.47%	1.31%
Volatility rate, minimum	15.00%	15.00%	15.00%
Volatility rate, maximum	23.00%	21.00%	20.00%
Ameren Missouri [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 4	$ 4	$ 4
Ameren Illinois Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	3	2	2
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 13	$ 12	$ 11

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Share-based Compensation Arrangement by Share-based Payment Award, Performance Period

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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