Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5		$ 3
Included in regulatory assets/liabilities			(1)
Total realized and unrealized gains (losses)	0		(1)
Purchases	3		7
Sales	(1)		(3)
Settlements	(2)		(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			1
Ending balance	5		5
Change in unrealized gains (losses) related to assets/liabilities still held	0		(1)
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0		(174)
Included in regulatory assets/liabilities	(1)		(27)
Total realized and unrealized gains (losses)	(1)		(27)
Purchases	1
Settlements			16
Transfers out of Level 3			185
Ending balance	0		0
Change in unrealized gains (losses) related to assets/liabilities still held	0		0
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(100)	[1]	81	[1]
Included in regulatory assets/liabilities	(15)		(175)	[1]
Total realized and unrealized gains (losses)	(15)		(175)	[1]
Purchases	40		21	[1]
Sales			(1)	[1]
Settlements	(15)		(22)	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	(3)
Transfers out of Level 3	4		(4)	[1]
Ending balance	(89)		(100)	[1]
Change in unrealized gains (losses) related to assets/liabilities still held	(25)		(175)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)		(1)
Included in regulatory assets/liabilities	(3)		(2)
Total realized and unrealized gains (losses)	(3)		(2)
Purchases	(2)
Settlements	1		1
Ending balance	(6)		(2)
Change in unrealized gains (losses) related to assets/liabilities still held	(2)		(1)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5		3
Included in regulatory assets/liabilities			(1)
Total realized and unrealized gains (losses)	0		(1)
Purchases	3		7
Sales	(1)		(3)
Settlements	(2)		(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			1
Ending balance	5		5
Change in unrealized gains (losses) related to assets/liabilities still held	0		(1)
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0		(14)
Included in regulatory assets/liabilities			(2)
Total realized and unrealized gains (losses)			(2)
Purchases
Settlements			1
Transfers out of Level 3			15
Ending balance	0		0
Change in unrealized gains (losses) related to assets/liabilities still held	0		0
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	11		21
Included in regulatory assets/liabilities	3		11
Total realized and unrealized gains (losses)	3		11
Purchases	40		21
Sales			(1)
Settlements	(36)		(37)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	(3)
Transfers out of Level 3	4		(4)
Ending balance	19		11
Change in unrealized gains (losses) related to assets/liabilities still held	(1)		0
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)		(1)
Included in regulatory assets/liabilities	(3)		(2)
Total realized and unrealized gains (losses)	(3)		(2)
Purchases	(2)
Settlements	1		1
Ending balance	(6)		(2)
Change in unrealized gains (losses) related to assets/liabilities still held	(2)		(1)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0		(160)
Included in regulatory assets/liabilities	(1)		(25)
Total realized and unrealized gains (losses)	(1)		(25)
Purchases	1
Settlements			15
Transfers out of Level 3			170
Ending balance	0		0
Change in unrealized gains (losses) related to assets/liabilities still held	0		0
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(111)		(140)
Included in regulatory assets/liabilities	(18)		(226)
Total realized and unrealized gains (losses)	(18)		(226)
Purchases
Sales
Settlements	21		255
Transfers out of Level 3
Ending balance	(108)		(111)
Change in unrealized gains (losses) related to assets/liabilities still held	$ (24)		$ (191)	[2]

[1] Ameren amounts include intercompany eliminations.

[2] The change in unrealized losses was due to decreases in long-term power prices applied to 20-year Ameren Illinois swap contracts, which expire in May 2032.