Short-Term Debt And Liquidity Short-Term Debt And Liquidity (Commercial Paper) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Commercial Paper
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 639	$ 54
Commercial paper outstanding	714	368
Weighted average interest rate	0.36%	0.56%
Peak short-term borrowings	910	368
Peak short-term borrowings interest rate	0.75%	0.85%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	110	0
Weighted average interest rate	0.38%	0.00%
Peak short-term borrowings	495	0
Peak short-term borrowings interest rate	0.70%	0.00%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	165	0
Weighted average interest rate	0.32%	0.00%
Peak short-term borrowings	300	0
Peak short-term borrowings interest rate	0.60%	0.00%
Ameren Illinois Company | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	32	0
Ameren Missouri [Member] | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	97	0
Ameren (parent) [Domain]
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	423	54
Weighted average interest rate	0.36%	0.56%
Peak short-term borrowings	625	368
Peak short-term borrowings interest rate	0.75%	0.85%
Ameren (parent) [Domain] | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 585	$ 368