Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 6	$ 9	$ 11	$ 16
Interest income on industrial development revenue bonds	[1]	6	7	13	14
Interest and dividend income	[1]	4	2	8	5
Other	[1]	0	3	3	4
Total miscellaneous income	[1]	16	21	35	39
Donations	[1]	2	1	10	6
Other	[1]	4	3	7	7
Total miscellaneous expense	[1]	6	4	17	13
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		5	8	9	15
Interest income on industrial development revenue bonds		6	7	13	14
Interest and dividend income		1	1	1	1
Total miscellaneous income		12	16	23	30
Donations		1	1	3	3
Other		1	1	2	3
Total miscellaneous expense		2	2	5	6
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1	2	1
Interest and dividend income		3	1	7	3
Other		0	3	2	4
Total miscellaneous income		4	5	11	8
Donations		1		4	3
Other		1	1	3	2
Total miscellaneous expense		$ 2	$ 1	$ 7	$ 5

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