Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 9	[1]	$ 8	[1]	$ 16	[1]	$ 16	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]	14	[1]	14	[1]
Interest and dividend income	2	[1]	1	[1]	5	[1]	1	[1]
Other	3	[1]		[1]	4	[1]		[1]
Total miscellaneous income	21	[1]	16	[1]	39	[1]	31	[1]
Donations	1	[1]	1	[1]	6	[1]	5	[1]
Other	3	[1]	4	[1]	7	[1]	8	[1]
Total miscellaneous expense	4	[1]	5	[1]	13	[1]	13	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	8		7		15		14
Interest income on industrial development revenue bonds	7		7		14		14
Interest and dividend income	1				1
Total miscellaneous income	16		14		30		28
Donations	1		1		3		3
Other	1		2		3		5
Total miscellaneous expense	2		3		6		8
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1		1		1		2
Interest and dividend income	1		1		3		1
Other	3				4
Total miscellaneous income	5		2		8		3
Donations					3		3
Other	1		1		2		1
Total miscellaneous expense	$ 1		$ 1		$ 5		$ 4

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.