Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 6	$ 8
Interest income on industrial development revenue bonds	[1]	7	7
Interest income	[1]	2	4
Other	[1]	0	1
Total miscellaneous income	[1]	15	20
Donations	[1]	5	5
Other	[1]	4	2
Total miscellaneous expense	[1]	9	7
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		5	7
Interest income on industrial development revenue bonds		7	7
Other		0	1
Total miscellaneous income		12	15
Donations		0	1
Other		2	1
Total miscellaneous expense		2	2
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1
Interest income		2	4
Total miscellaneous income		3	5
Donations		4	4
Other		2	1
Total miscellaneous expense		$ 6	$ 5

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