Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Operating revenues		$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 6,076	$ 6,098	$ 6,053
Operating expenses										4,695	4,839	4,799
Operating loss		145	691	325	220	140	626	237	256	1,381	1,259	1,254
Interest income from affiliates	[1],[2]									13	14	10
Other Nonoperating Income (Expense)										42	44	57
Interest charges										382	355	341
Income taxes (benefit)										382	363	377
Net income attributable to Ameren common shareholders - continuing operations		32	369	147	105	30	343	98	108	653	579	587
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations		0	0	0	0	(1)	0	52	0	0	51	(1)
Net income attributable to Ameren common shareholders		$ 32	$ 369	$ 147	$ 105	$ 29	$ 343	$ 150	$ 108	653	630	586
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)										(20)	6	(12)
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders										633	585	575
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders										0	51	(1)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders										633	636	574
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)										(7)	3	(7)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating expenses										14	14	11
Operating loss										(14)	(14)	(11)
Equity in earnings of subsidiaries										663	600	607
Interest income from affiliates										10	6	3
Other Nonoperating Income (Expense)										(5)	(5)	2
Interest charges										28	3	16
Income taxes (benefit)										(27)	5	(2)
Net income attributable to Ameren common shareholders - continuing operations										653	579	587
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations										0	51	(1)
Net income attributable to Ameren common shareholders										653	630	586
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)										(20)	6	(12)
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders										633	585	575
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders										0	51	(1)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders										633	636	574
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)										$ (7)	$ 3	$ (7)

[1] Includes Ameren Illinois' interest income on the IEIMA revenue requirement reconciliation adjustment regulatory assets.

[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.