Consolidated Statement of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 290	$ 371	$ 588	$ 626
Other Comprehensive Income (Loss), Net of Taxes
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	(1)	2	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	290	370	590	627
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income Attributable to Ameren Common Shareholders	$ 288	$ 368	$ 585	$ 622