Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,402	$ 1,723	$ 1,538	$ 1,514	$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 6,177	$ 6,076	$ 6,098
Operating expenses									4,719	4,695	4,839
Operating loss	225	581	398	254	145	691	325	220	1,458	1,381	1,259
Interest income from affiliates									8	13	14
Other Nonoperating Income (Expense)									38	42	44
Interest charges									391	382	355
Income taxes (benefit)									576	382	363
Net income attributable to Ameren common shareholders - continuing operations									523	653	579
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations									0	0	51
Net income attributable to Ameren common shareholders	$ (60)	$ 288	$ 193	$ 102	$ 32	$ 369	$ 147	$ 105	523	653	630
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									5	(20)	6
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders									528	633	585
Comprehensive Income from Discontinued Operations Attributable to Ameren Common shareholders									0	0	51
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									528	633	636
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									3	(7)	3
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	0
Operating expenses									13	14	14
Operating loss									(13)	(14)	(14)
Equity in earnings of subsidiaries									659	663	600
Interest income from affiliates									9	10	6
Other Nonoperating Income (Expense)									0	(5)	(5)
Interest charges									31	28	3
Income taxes (benefit)									101	(27)	5
Net income attributable to Ameren common shareholders - continuing operations									523	653	579
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations									0	0	51
Net income attributable to Ameren common shareholders									523	653	630
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									5	(20)	6
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders									528	633	585
Comprehensive Income from Discontinued Operations Attributable to Ameren Common shareholders									0	0	51
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									528	633	636
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									$ 3	$ (7)	$ 3