Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Total operating revenues	$ 5,794	$ 5,910	$ 6,291
Operating expenses	4,494	4,643	4,934
Operating Income	1,300	1,267	1,357
Interest income from affiliates	4	8	7
Total other expense, net	151	130	102
Interest Charges	(419)	(381)	(401)
Income tax benefit	155	182	237
Comprehensive Income from Continuing Operations
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	16	5	(4)
Comprehensive Income Attributable to Ameren Common Shareholders	887	833	811
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement benefit plan activity, tax (benefit)	5	1	(1)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Total operating revenues	0	0	0
Operating expenses	12	15	11
Operating Income	(12)	(15)	(11)
Equity in earnings of subsidiaries	908	850	857
Interest income from affiliates	4	5	3
Total other expense, net	(8)	(2)	(12)
Interest Charges	(57)	(39)	(34)
Income tax benefit	36	29	12
Net Income Available to Common Shareholder	871	828	815
Comprehensive Income from Continuing Operations
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	16	5	(4)
Comprehensive Income Attributable to Ameren Common Shareholders	887	833	811
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement benefit plan activity, tax (benefit)	$ 5	$ 1	$ (1)