Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5	$ 5
Included in regulatory assets/liabilities	(2)
Purchases		1
Settlements	(2)	(1)
Ending balance	1	5
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	(2)
Settlements	1
Ending balance	(5)	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Included in regulatory assets/liabilities		1
Purchases	(2)	1
Settlements	2
Ending balance	0	2
Change in unrealized gains (losses) related to assets/liabilities held at period end
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(89)	(100)
Included in regulatory assets/liabilities	(17)	19
Settlements	(5)	3
Transfers into Level 3		(2)
Transfer out of Level 3	1	1
Ending balance	(110)	(79)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(15)	11
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5	5
Included in regulatory assets/liabilities	(2)
Purchases		1
Settlements	(2)	(1)
Ending balance	1	5
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	(2)
Settlements	1
Ending balance	(5)	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance
Included in regulatory assets/liabilities
Purchases
Ending balance
Change in unrealized gains (losses) related to assets/liabilities held at period end
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19	11
Included in regulatory assets/liabilities	(5)	5
Settlements	(5)	(13)
Transfers into Level 3		(2)
Transfer out of Level 3	1	1
Ending balance	10	2
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)	(3)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Included in regulatory assets/liabilities		1
Purchases	(2)	1
Settlements	2
Ending balance	0	2
Change in unrealized gains (losses) related to assets/liabilities held at period end
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(108)	(111)
Included in regulatory assets/liabilities	(12)	14
Settlements		16
Transfers into Level 3
Transfer out of Level 3
Ending balance	(120)	(81)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (14)	$ 14