Retirement Benefits (Narrative) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2015 USD ($) bond partnership	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ 567	$ 710
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate		0.50%
Number of high-quality corporate bonds | bond		700
Defined benefit plan, estimated future employer contributions during the next five years		$ 280
Amortization basis, straight line, in years		10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ (544)	(616)
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in future years		7.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ (23)	$ (94)
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		$ 40
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years		$ 70
Private equity
Defined Benefit Plan Disclosure [Line Items]
Number of limited partnerships in private equity funds | partnership		8
Minimum invested capital within limited partnership investments		$ 1
Maximum invested capital within limited partnership investments		$ 4
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		40.00%
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		50.00%

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