Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 bond		Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ 710	[1]	$ 461	[1]
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.75%
Number of high-quality corporate bonds	700
Defined benefit plan, estimated future employer contributions during the next five years	290
Actual return in excess of (or less than) expected return, percentage	25.00%
Expected return on plan assets, period	4 years
Amortization basis, straight line, in years	10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	(616)	[1]	(439)	[1]
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in future years	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	(94)	[1]	(22)	[1]
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in future years	7.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	25
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	115
Private equity
Defined Benefit Plan Disclosure [Line Items]
Number of limited partnerships in private equity funds	9
Minimum invested capital within limited partnership investments	0.1
Maximum invested capital within limited partnership investments	$ 5
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	41.00%
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	40.00%

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.