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Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2014
Jun. 30, 2014
May. 31, 2014
Apr. 30, 2014
Jan. 31, 2014
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Mar. 31, 2016
May. 15, 2014
Oct. 31, 2013
Long-Term Debt And Equity Financings [Line Items]                          
Debt Default Provision Excess               $ 25,000,000          
Preferred stock, authorized           100,000,000   100,000,000          
Preferred stock, par value           $ 0.01   $ 0.01          
Preferred stock, shares outstanding           0   0          
Common stock, shares authorized 400,000,000         400,000,000   400,000,000 400,000,000        
Repayments of Other Long-term Debt               $ 120,000,000 $ 697,000,000 $ 399,000,000      
Union Electric Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Preferred stock, authorized           7,500,000.0   7,500,000.0          
Preferred stock, par value           $ 1   $ 1          
Common stock, shares authorized 150,000,000         150,000,000   150,000,000 150,000,000        
Repayments of Other Long-term Debt               $ 120,000,000 $ 109,000,000 249,000,000      
Ameren Illinois Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Preferred stock, authorized           2,600,000.0   2,600,000.0          
Common stock, shares authorized 45,000,000         45,000,000   45,000,000 45,000,000        
Repayments of Other Long-term Debt               $ 0 $ 163,000,000 150,000,000      
Debt instrument face amount [1]         $ 1,000,000                
Common stock equity to capitalization ratio           51.00%   51.00%          
Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds                          
Long-Term Debt And Equity Financings [Line Items]                          
Redemptions of long-term debt         163,000,000                
Parent Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Common stock, shares authorized     8,600,000.0                   4,000,000
Repayments of Other Long-term Debt               $ 0 425,000,000 $ 0      
Ameren Missouri and Ameren Illinois                          
Long-Term Debt And Equity Financings [Line Items]                          
Bonds interest rate assumption           5.00%   5.00%          
Dividend rate on preferred shares, percentage               6.00%          
Senior Unsecured Notes270 due 2020 [Member] | Parent Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Long-term debt interest rate           2.70%   2.70%          
Senior Unsecured Notes270 due 2020 [Member] | Parent Company | Unsecured Debt [Member]                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount $ 0         $ 350,000,000   $ 350,000,000 0        
Long-term debt interest rate           2.70%   2.70%          
Proceeds from issuance of secured debt           $ 348,000,000              
Senior Unsecured Notes365 due 2026 [Member] | Parent Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Long-term debt interest rate           3.65%   3.65%          
Senior Unsecured Notes365 due 2026 [Member] | Parent Company | Unsecured Debt [Member]                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount 0         $ 350,000,000   $ 350,000,000 0        
Long-term debt interest rate           3.65%   3.65%          
Proceeds from issuance of secured debt           $ 347,000,000              
Series1993570 Due2024 [Member] | Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [2]           1,000,000   $ 1,000,000          
Senior Unsecured Notes8875 Due2014 [Member] | Parent Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Repayments of Other Long-term Debt     $ 425,000,000                    
Long-term debt interest rate     8.875%                    
5.40% Senior secured notes due 2016 | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [3] 260,000,000         $ 260,000,000   $ 260,000,000 260,000,000        
Long-term debt interest rate           5.40%   5.40%          
Senior Secured Notes, 3.65%, Due 2045 [Member] | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount 0 [3]         $ 250,000,000   $ 250,000,000 0 [3]        
Long-term debt interest rate           3.65%   3.65%          
Proceeds from issuance of secured debt             $ 247,000,000            
Senior Secured Notes 4.75% Due2015 [Member] | Union Electric Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Repayments of Other Long-term Debt             $ 114,000,000            
Long-term debt interest rate             4.75%            
Senior Secured Notes 4.75% Due2015 [Member] | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [3] 114,000,000         $ 0   $ 0 114,000,000        
Long-term debt interest rate           4.75%   4.75%          
Senior Secured Notes, 3.50%, Due 2024 [Member] | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount       $ 350,000,000                  
Long-term debt interest rate       3.50%                  
Proceeds from issuance of secured debt       $ 348,000,000                  
5.50% Senior secured notes due 2014 | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Long-term debt interest rate                       5.50%  
Redemptions of long-term debt     $ 104,000,000                    
Senior Secured Notes 4.80% Due 2043 | Ameren Illinois Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [4] 280,000,000         $ 280,000,000   $ 280,000,000 280,000,000        
Long-term debt interest rate           4.80%   4.80%          
1993 5.45% Series due 2028 | Union Electric Company | Environmental Improvement And Pollution Control Revenue Bonds                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [2]           $ 1,000,000   $ 1,000,000          
Long-term debt interest rate           5.45%   5.45%          
Senior Secured Notes, 4.15%, Due 2046 [Member] | Ameren Illinois Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount 0 [4]         $ 250,000,000   $ 250,000,000 0 [4]        
Long-term debt interest rate           4.15%   4.15%          
Proceeds from issuance of secured debt           $ 245,000,000              
Senior Secured Notes, 4.30%, Due 2044 [Member] [Member] | Ameren Illinois Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount   $ 250,000,000                      
Long-term debt interest rate   4.30%                      
Proceeds from issuance of secured debt   $ 246,000,000                      
Senior Secured Notes, 3 Point 25, Due 2025 [Member] | Ameren Illinois Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [4] 300,000,000         $ 300,000,000 [5]   $ 300,000,000 [5] 300,000,000        
Long-term debt interest rate           3.25%   3.25%          
Proceeds from issuance of secured debt 298,000,000                        
Senior Secured Notes, 2.70%, Due 2022 | Ameren Illinois Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [4],[5] 400,000,000         $ 400,000,000   $ 400,000,000 400,000,000        
Long-term debt interest rate           2.70%   2.70%          
3.90% Senior secured notes due 2042 | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [3],[6] 485,000,000         $ 485,000,000   $ 485,000,000 485,000,000        
Long-term debt interest rate           3.90%   3.90%          
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [2]           $ 1,000,000   $ 1,000,000          
Long-term debt interest rate           5.90%   5.90%          
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount [7] $ 1,000,000         $ 1,000,000 [2]   $ 1,000,000 [2] $ 1,000,000        
Long-term debt interest rate           5.90%   5.90%          
Redemptions of long-term debt [8]         $ 32,000,000                
Minimum | Ameren Illinois Company                          
Long-Term Debt And Equity Financings [Line Items]                          
Common stock equity to capitalization ratio           30.00%   30.00%          
Subsequent Event | 5.40% Senior secured notes due 2016 | Union Electric Company | Secured Debt                          
Long-Term Debt And Equity Financings [Line Items]                          
Debt instrument face amount                     $ 260,000,000    
Long-term debt interest rate                     5.40%    
[1] These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding.
[2] These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[3] These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2042.
[4] These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2024.
[5] Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur, and so long as any of the 9.75% senior secured notes due 2018, 6.25% senior secured notes due 2018 and 6.125% senior secured notes due 2017 are outstanding, Ameren Illinois will not optionally redeem, purchase or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions; therefore, a release date will not occur so long as any of these notes remain outstanding.
[6] Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019, 6.00% senior secured notes due 2018 and 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions.
[7] These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture. They are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[8] Less than $1 million principal amount of the bonds remains outstanding after redemption.