Related Party Transactions (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended				6 Months Ended
	May 31, 2015 Marketing Company [Member]	May 31, 2013 Marketing Company [Member]	May 31, 2015 Ameren Missouri [Member]	May 31, 2014 Ameren Missouri [Member]	Jun. 30, 2012 Ameren Illinois Company [Member] Maximum [Member]	Jun. 30, 2012 Genco [Member]	Mar. 28, 2012 Genco [Member]
Related Party Transaction [Line Items]
Purchase price							$ 100
Put Option Premium							2.5
Intercompany receivable						1
Energy Supply Agreements, Amount	4	1	3	1
Payable to Marketing Company for the purchase of trade receivables					1
Purchase of receivables from Marketing Company					$ 1