Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 8	$ 5
Interest income on industrial development revenue bonds	[1]	7	7
Interest and dividend income	[1]	4	4
Other	[1]	1	3
Total miscellaneous income	[1]	20	19
Donations	[1]	5	8
Other	[1]	2	3
Total miscellaneous expense	[1]	7	11
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		7	4
Interest income on industrial development revenue bonds		7	7
Other		1	0
Total miscellaneous income		15	11
Donations		1	2
Other		1	1
Total miscellaneous expense		2	3
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1
Interest and dividend income		4	4
Other		0	2
Total miscellaneous income		5	7
Donations		4	3
Other		1	2
Total miscellaneous expense		$ 5	$ 5

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