Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 bond		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.75%
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	$ 19	[1],[2]
Defined Benefit Plan, Funded Status of Plan	461	[3]	1,143	[3]
Number of high-quality corporate bonds	500
Defined benefit plan, estimated future employer contributions during the next five years	270
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, Recognized Net Gain (Loss) Due to Curtailments	12	[2]
Defined Benefit Plan, Funded Status of Plan	(439)	[3]	(924)	[3]
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in 2013	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	7	[2]
Defined Benefit Plan, Funded Status of Plan	(22)	[3]	(219)	[3]
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in 2013	7.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	20
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	100
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	52.00%
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	47.00%

[1] (b)The net periodic benefit cost includes a $6 million and a $7 million net gain for pension benefits and postretirement benefits, respectively, which was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). This net gain includes the curtailment gain recognized in 2013 as a result of a significant reduction in employees as of the December 2, 2013 closing date of the New AER divestiture. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.

[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries

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