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, 2015	Mar. 31, 2014
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (6)	$ 5
Included in regulatory assets/liabilities	(1)	(2)
Settlements	1	(2)
Ending balance	(6)	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	(3)	(1)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	(6)
Included in regulatory assets/liabilities	1
Settlements		1
Ending balance	(1)	(5)
Change in unrealized gains (losses) related to assets/liabilities held at period end	1
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)	0
Purchases	1	(2)
Settlements		2
Ending balance	0	0
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(133)	(89)
Included in regulatory assets/liabilities	(27)	(17)
Settlements		(5)
Transfer out of Level 3		(1)
Ending balance	(160)	(110)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(24)	(15)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	5
Included in regulatory assets/liabilities	(1)	(2)
Settlements	1	(2)
Ending balance	(6)	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	(3)	(1)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	(6)
Included in regulatory assets/liabilities	1
Settlements		1
Ending balance	(1)	(5)
Change in unrealized gains (losses) related to assets/liabilities held at period end	1
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)	0
Purchases	0	0
Settlements		0
Ending balance	(1)	0
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	9	19
Included in regulatory assets/liabilities	(2)	(5)
Settlements	(3)	(5)
Transfer out of Level 3		(1)
Ending balance	4	10
Change in unrealized gains (losses) related to assets/liabilities held at period end		(1)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Purchases	1	(2)
Settlements		2
Ending balance	1	0
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(142)	(108)
Included in regulatory assets/liabilities	(25)	(12)
Settlements	3
Ending balance	(164)	(120)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (24)	$ (14)