Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	[1]	$ 92	[2]	$ 79	[2]	$ 91
Interest cost	[1]	174	[2]	183	[2]	163
Expected return on plan assets	[1]	(248)		(229)		(218)
Prior service credit	[1]	(1)		(1)		(2)
Actuarial loss	[1]	74		49		87
Curtailment	[1]					(12)
Settlement Loss	[1]	1
Net periodic benefit cost	[1]	92		81		109	[3]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	[1]	24	[2]	19	[2]	22
Interest cost	[1]	48	[2]	50	[2]	46
Expected return on plan assets	[1]	(68)		(65)		(62)
Prior service credit	[1]	(5)		(5)		(6)
Actuarial loss	[1]	5		(7)		8
Curtailment	[1]					(7)
Settlement Loss	[1]			0
Net periodic benefit cost	[1]	$ 4		$ (8)		1	[3]
New Ameren Energy Resources Company, LLC | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1],[3]					6
New Ameren Energy Resources Company, LLC | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1],[3]					$ 7

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries

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

[3] (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.