Summary Of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2015

Accounting Policies [Abstract]

Schedule of Change in Asset Retirement Obligation [Table Text Block]

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2015:

	Ameren Missouri		Ameren Illinois(a)		Ameren
Balance at December 31, 2014	$	389	$	7	$	396
Accretion in 2015(b)	5		(c)		5
Change in estimates(d)	99		(c)		99
Balance at March 31, 2015	$	493	$	7	$	500

(a) Included in “Other deferred credits and liabilities” on the balance sheet.

(b) Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

(c) Less than $1 million.

(d) The ARO increase also resulted in a corresponding increase recorded to “Property and Plant, Net.” Ameren Missouri’s estimates related to its Callaway energy center decommissioning costs changed to reflect increased costs from the 2015 cost study and funding analysis, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information.

Summary Of Nonvested Shares Related To Long-Term Incentive Plan

A summary of nonvested performance share units at March 31, 2015, and changes during the three months ended March 31, 2015, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:

	Number of Performance Share Units		Weighted-average Fair Value Per Performance Share Unit
Nonvested at January 1, 2015	1,162,377		$	35.35
Granted(a)	566,332		52.88
Forfeitures	(1,944	)	34.75
Vested(b)	(68,411	)	47.88
Nonvested at March 31, 2015	1,658,354		$	40.82

(a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan.

(b) Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.

Schedule of Unrecognized Tax Benefits

The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of March 31, 2015, and December 31, 2014:

	March 31, 2015			December 31, 2014
Ameren	$	53		$	54
Ameren Missouri	(1		)	—
Ameren Illinois	(1		)	(1		)

Schedule of Unrecognized Tax Benefits That Would Impact Effective Tax Rate

The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that, if recognized, would impact results of operations as of March 31, 2015, and December 31, 2014:

	March 31, 2015			December 31, 2014
Ameren	$	52		$	52
Ameren Missouri	(1		)	—
Ameren Illinois	(1		)	(1		)

Schedule of Excise Taxes

The following table presents excise taxes recorded in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” for the three months ended March 31, 2015 and 2014:

	Three Months
	2015		2014
Ameren Missouri	$	34	$	34
Ameren Illinois	23		26
Ameren	$	57	$	60