Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	[1]	$ 18	$ 19
Derivative liabilities	[2]	264	232
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets			2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		22	22
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		7	7
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		1	2
Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		1
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		38	37
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		26	19
Power | Other current assets
Derivative [Line Items]
Derivative assets		16	15
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities		13	14
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		157	131
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities		1	2
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	17	18
Derivative liabilities	[2]	44	46
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets			2
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		22	22
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		7	7
Union Electric Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets			1
Union Electric Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		6	6
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		8	6
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets		16	15
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities			3
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities		1	2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	1	1
Derivative liabilities	[2]	220	186
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		1	1
Ameren Illinois Company | Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		32	31
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		18	13
Ameren Illinois Company | Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		13	11
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		$ 157	$ 131

[1] Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.

[2] Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.