Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Revenues	$ 4	$ 4	$ 8	$ 8
Ameren Missouri [Member] | Other Operations And Maintenance [Member]
Related Party Transaction [Line Items]
Operating Expenses	27	28	55	59
Ameren Missouri [Member] | Rent and Facility Services [Member]
Related Party Transaction [Line Items]
Operating Revenues	4	4	8	8
Ameren Missouri [Member] | Support Services Agreement [Member] | Other Operations And Maintenance [Member]
Related Party Transaction [Line Items]
Operating Expenses	27	28	55	59
Ameren Missouri [Member] | Money Pool Borrowings (Advances)[Member]
Related Party Transaction [Line Items]
Interest Charges
Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues		1		1
Ameren Illinois Company [Member] | Other Operations And Maintenance [Member]
Related Party Transaction [Line Items]
Operating Expenses	22	21	45	45
Ameren Illinois Company [Member] | Affiliated Entity [Member] | Purchased Power [Member]
Related Party Transaction [Line Items]
Operating Expenses	72	48	160	94
Ameren Illinois Company [Member] | Power Supply Agreements [Member] | Marketing Company [Member] | Purchased Power [Member]
Related Party Transaction [Line Items]
Operating Expenses	72	48	160	94
Ameren Illinois Company [Member] | Support Services Agreement [Member] | Other Operations And Maintenance [Member]
Related Party Transaction [Line Items]
Operating Expenses	22	21	45	45
Ameren Illinois Company [Member] | Money Pool Borrowings (Advances)[Member]
Related Party Transaction [Line Items]
Interest Charges
Genco [Member]
Related Party Transaction [Line Items]
Operating Revenues	193	242	386	482
Genco [Member] | Other Operations And Maintenance [Member]
Related Party Transaction [Line Items]
Operating Expenses	5	4	10	10
Genco [Member] | Affiliated Entity [Member] | Purchased Power [Member]
Related Party Transaction [Line Items]
Operating Expenses		12		12
Genco [Member] | Power Supply Agreements [Member] | Marketing Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	193	242	386	482
Genco [Member] | Power Supply Agreements [Member] | EEI [Member] | Purchased Power [Member]
Related Party Transaction [Line Items]
Operating Expenses		12		12
Genco [Member] | Support Services Agreement [Member] | Other Operations And Maintenance [Member]
Related Party Transaction [Line Items]
Operating Expenses	5	4	10	10
Genco [Member] | Money Pool Borrowings (Advances)[Member]
Related Party Transaction [Line Items]
Interest Charges