Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 1,399	$ 1,723	$ 1,537	$ 1,515	$ 6,291	$ 6,174	$ 6,076
Operating expenses									4,934	4,764	4,754
Operating loss	166	533	385	273	212	569	387	242	1,357	1,410	1,322
Interest income from affiliates									7	8	13
Other Nonoperating Income (Expense)									102	86	101
Interest charges									401	391	382
Income taxes (benefit)									237	576	382
Net Income (Loss) Available to Common Stockholders, Basic	$ 68	$ 357	$ 239	$ 151	$ (60)	$ 288	$ 193	$ 102	815	523	653
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									(4)	5	(20)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									811	528	633
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									(1)	3	(7)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	0
Operating expenses									11	15	19
Operating loss									(11)	(15)	(19)
Equity in earnings of subsidiaries									857	659	663
Interest income from affiliates									3	9	10
Other Nonoperating Income (Expense)									(12)	2	0
Interest charges									34	31	28
Income taxes (benefit)									(12)	101	(27)
Net income attributable to Ameren common shareholders - continuing operations									815	523	653
Net Income (Loss) Available to Common Stockholders, Basic									815	523	653
Comprehensive Income from Continuing Operations
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									811	528	633
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									$ (1)	$ 3	$ (7)