Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 539		$ 453
Tax credit carryforwards	130		144
Deferred Tax Assets, Charitable Contribution Carryforwards	18	[1]	10
Deferred Tax Assets, Valuation Allowance, Noncurrent	(11)	[2]	(4)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	7		6
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	494	[3]	407
Tax credit carryforwards	109	[3]	105
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	45	[3]	46
Tax credit carryforwards	21	[1]	41
Tax Credit Carryforward, Valuation Allowance			(2)
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	37		39
Tax credit carryforwards	29		26
Deferred Tax Assets, Charitable Contribution Carryforwards	0	[1]	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	[2]	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0		0
Union Electric Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	33	[3]	35
Tax credit carryforwards	29	[3]	26
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4	[3]	4
Tax credit carryforwards	0	[1]	0
Tax Credit Carryforward, Valuation Allowance			0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	137		131
Tax credit carryforwards	1		2
Deferred Tax Assets, Charitable Contribution Carryforwards	0	[1]	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	[2]	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0		0
Ameren Illinois Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	137	[3]	127
Tax credit carryforwards	1	[3]	1
Ameren Illinois Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	[3]	4
Tax credit carryforwards	0	[1]	1
Tax Credit Carryforward, Valuation Allowance			0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	365		283
Tax credit carryforwards	100		116
Deferred Tax Assets, Charitable Contribution Carryforwards	18	[1]	10
Deferred Tax Assets, Valuation Allowance, Noncurrent	(11)	[2]	(4)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	7		6
Other | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	324	[3]	245
Tax credit carryforwards	79	[3]	78
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	41	[3]	38
Tax credit carryforwards	$ 21	[1]	40
Tax Credit Carryforward, Valuation Allowance			$ (2)
Minimum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2029
Tax credit carryforward, expiration period start	Jan. 01, 2029
Minimum | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2029
Tax credit carryforward, expiration period start	Jan. 01, 2017
Maximum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2036
Tax credit carryforward, expiration period start	Jan. 01, 2036
Maximum | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2036
Tax credit carryforward, expiration period start	Jan. 01, 2021

[1] Will expire between 2017

[2] See Schedule II under Part IV, Item 15, in this report for information on changes in the valuation allowance.

[3] Will expire between 2029 and 2036.