Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (Not Designated As Hedging Instrument, USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 34	[1]	$ 29	[1]
Derivative liabilities	178	[1]	230	[1]
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	6	[1]	8	[1]
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	3	[1]	4	[1]
Fuel Oils | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	2	[1]	2	[1]
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	[1]	2	[1]
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	2	[1]	1	[1]
Natural Gas | Other assets
Derivative [Line Items]
Derivative assets			1	[1]
Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	32	[1]	64	[1]
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	25	[1]	45	[1]
Power | Other current assets
Derivative [Line Items]
Derivative assets	23	[1]	14	[1]
Power | Other assets
Derivative [Line Items]
Derivative assets			1	[1]
Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	13	[1]	25	[1]
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	99	[1]	90	[1]
Uranium | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	5	[1]	1	[1]
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	[1]	1	[1]
Union Electric Company
Derivative [Line Items]
Derivative assets	33	[1]	28	[1]
Derivative liabilities	24	[1]	25	[1]
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	6	[1]	8	[1]
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	3	[1]	4	[1]
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	[1]	2	[1]
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	[1]	2	[1]
Union Electric Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	1	[1]		[1]
Union Electric Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets			1	[1]
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	5	[1]	8	[1]
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	6	[1]	7	[1]
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	23	[1]	14	[1]
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative assets			1	[1]
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	[1]	4	[1]
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	5	[1]	1	[1]
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	[1]	1	[1]
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Derivative liabilities	154	[1]	205	[1]
Ameren Illinois Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets		[1]		[1]
Ameren Illinois Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets		[1]		[1]
Ameren Illinois Company | Fuel Oils | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Ameren Illinois Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Ameren Illinois Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets				[1]
Ameren Illinois Company | Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	27	[1]	56	[1]
Ameren Illinois Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	19	[1]	38	[1]
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative assets		[1]		[1]
Ameren Illinois Company | Power | Other assets
Derivative [Line Items]
Derivative assets				[1]
Ameren Illinois Company | Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	9	[1]	21	[1]
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	99	[1]	90	[1]
Ameren Illinois Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		[1]		[1]

[1] ncludes derivatives subject to regulatory deferral.