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) - Power - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (187)	$ (190)	$ (188)	$ (178)
Included in regulatory assets/liabilities	(1)	(10)
Settlements	2	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	(11)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	4	4	7	7
Included in regulatory assets/liabilities	(2)	0
Settlements	(1)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)	0
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(191)	(194)	$ (195)	$ (185)
Included in regulatory assets/liabilities	1	(10)
Settlements	3	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ 1	$ (11)