Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Operating revenues		$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 1,370	$ 1,670	$ 1,419	$ 1,594	$ 6,098	$ 6,053	$ 5,838
Operating expenses										4,839	4,799	4,654
Operating loss		140	626	237	256	125	561	322	246	1,259	1,254	1,184
Interest income from affiliates	[1],[2]									14	10	3
Other Nonoperating Income (Expense)										44	57	43
Interest charges										355	341	398
Income taxes (benefit)										363	377	311
Net income attributable to Ameren common shareholders - continuing operations		30	343	98	108	46	294	150	97	579	587	512
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations		(1)	0	52	0	2	(1)	(1)	(1)	51	(1)	(223)
Net income attributable to Ameren common shareholders		$ 29	$ 343	$ 150	$ 108	$ 48	$ 293	$ 149	$ 96	630	586	289
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)										6	(12)	30
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders										585	575	542
Other Comprehensive Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest											0	(18)
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders										51	(1)	(242)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders										636	574	300
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)										3	(7)	16
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating expenses										14	11	26
Operating loss										(14)	(11)	(26)
Equity in earnings of subsidiaries										600	607	546
Interest income from affiliates										6	3	3
Other Nonoperating Income (Expense)										(5)	2	(5)
Interest charges										3	16	42
Income taxes (benefit)										5	(2)	(36)
Net income attributable to Ameren common shareholders - continuing operations										579	587	512
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations										51	(1)	(223)
Net income attributable to Ameren common shareholders										630	586	289
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)										6	(12)	30
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders										585	575	542
Other Comprehensive Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest										0	0	(19)
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders										51	(1)	(242)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders										636	574	300
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)										$ 3	$ (7)	$ 16

[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.