Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets	[1]	$ 27	$ 18
Derivative liabilities	[2]	211	264
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets		2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5	22
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities			7
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		12	1
Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		3
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		4	38
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		10	26
Power | Other current assets
Derivative [Line Items]
Derivative assets		9	16
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities		15	13
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		173	157
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities			1
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	14	17
Derivative liabilities	[2]	18	44
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets		2
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	[1]	1
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5	22
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities			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	1	[1]
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		1	6
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		5	8
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets		9	16
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
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	[1]	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	13	1
Derivative liabilities	[2]	193	220
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		11	1
Ameren Illinois Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		2
Ameren Illinois Company | Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		3	32
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		5	18
Ameren Illinois Company | Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		12	13
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		$ 173	$ 157

[1] The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.

[2] The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.