Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		$ 642	$ 644
Liabilities settled	[1]	(1)
Accretion		7
Change of estimates		(9)
Other current liabilities		285	326
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		638	640
Liabilities settled	[1]	(1)
Accretion		7
Change of estimates		(9)
Other current liabilities		99	103
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		4	4
Liabilities settled	[1]	(1)
Accretion	[1]	1
Change of estimates		0
Other current liabilities		141	177
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities		6	6
Asset Retirement Obligation Balance | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities		$ 6	$ 6

[1] (c)Less than $1 million.