Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,316	$ 1,659	$ 1,379	$ 1,556	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 5,910	$ 6,291	$ 6,174
Operating expenses									4,643	4,934	4,764
Operating loss	$ 179	$ 520	$ 280	$ 288	$ 166	$ 533	$ 385	$ 273	1,267	1,357	1,410
Interest income from affiliates									8	7	8
Total other income (expense), net									130	102	86
Interest charges									381	401	391
Income taxes (benefit)									182	237	576
Comprehensive Income from Continuing Operations
Pension and other postretirement benefit plan activity, net of income taxes (benefit)									5	(4)	5
Comprehensive Income Attributable to Ameren Common Shareholders									833	811	528
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement benefit plan activity, tax (benefit)									1	(1)	3
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	0
Operating expenses									15	11	15
Operating loss									(15)	(11)	(15)
Equity in earnings of subsidiaries									850	857	659
Interest income from affiliates									5	3	9
Total other income (expense), net									(2)	(12)	2
Interest charges									39	34	31
Income taxes (benefit)									(29)	(12)	101
Net Income Attributable to Ameren Common Shareholders									828	815	523
Comprehensive Income from Continuing Operations
Pension and other postretirement benefit plan activity, net of income taxes (benefit)									5	(4)	5
Comprehensive Income Attributable to Ameren Common Shareholders									833	811	528
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement benefit plan activity, tax (benefit)									$ 1	$ (1)	$ 3