Consolidated Statement of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 194	$ 148	$ 298	$ 255
Other Comprehensive Income, Net of Taxes
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	2	4	2	2
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	196	152	300	257
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	1	1	3	3
Comprehensive Income Attributable to Ameren Common Shareholders	$ 195	$ 151	$ 297	$ 254