Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Revenues			$ 1.0				$ 1.0
Operating Expenses	28		27		60		55
Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	2.0				2.0
Operating Expenses	24		22		48		45
Ameren Missouri Power Supply Agreements with Ameren Illinois [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Revenues	1.0	[1]	1.0	[1]	1.0		1.0	[1]
Ameren Missouri and Ameren Illinois Rent and Facility Services [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Revenues	5.0		5.0		11.0		9.0
Ameren Missouri and Ameren Illinois Rent and Facility Services [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	1.0	[1]	1.0	[1]	1.0	[1]	1.0	[1]
Ameren Missouri and Genco Gas Transportation Agreement [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Revenues	1.0	[1]	1.0	[1]	1.0	[1]	1.0	[1]
Ameren Illinois Transmission Services Agreement with Marketing Company [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	7.0		3.0		13.0		5.0
Ameren Illinois Power Supply Agreements with Marketing Company [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	22		72		48		160
Purchased Power [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	22		72		49		160
Ameren Services Support Services Agreement [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Expenses	28		27		60		55
Ameren Services Support Services Agreement [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	24		22		48		45
Ameren Illinois Power Supply Agreements with Ameren Missouri [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	1	[1]	1	[1]	1		1	[1]
Related Party Money Pool Interest [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Interest Charges		[1]			1	[1]
Related Party Money Pool Interest [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Interest Charges	1	[1]	1	[1]	1	[1]	1	[1]
Insurance Premiums [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Expenses	1	[1],[2]	1	[1],[2]	1	[1],[2]	1	[1],[2]
Related Party One [Member] | Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Revenues	5.0		5.0		12.0		9.0
Related Party One [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	$ 7.0		$ 3.0		$ 13.0		$ 5.0

[1] Amount less than $1 million.

[2] Represents insurance premiums paid to Missouri Energy Risk Assurance Company, an affiliate, for replacement power.