Consolidated Statement Of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Other Paid-In Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-In Capital	Union Electric Company Preferred Stock Not Subject To Mandatory Redemption	Union Electric Company Retained Earnings	Ameren Illinois Company	Ameren Illinois Company Common Stock	Ameren Illinois Company Other Paid-In Capital	Ameren Illinois Company Preferred Stock Not Subject To Mandatory Redemption	Ameren Illinois Company Retained Earnings	Ameren Illinois Company Deferred Retirement Benefit Costs	Ameren Illinois Company Accumulated Other Comprehensive Income (Loss)
Beginning of year at Dec. 31, 2014			$ 5,617	$ 1,103	$ (9)		$ 142				$ 1,569		$ 1,892			$ 1,980		$ 611	$ 8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(1)
Net income attributable to Ameren common shareholders	$ 630			630
Net income	636								$ 355				355	$ 217				217
Common stock dividends				(402)									(575)					0
Capital contribution from parent									224		253			25		25
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ (6)				6									3						$ (3)
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2015	242.6
End of year at Dec. 31, 2015	$ 7,088	$ 2	5,616	1,331	(3)	$ (3)	142			$ 511	1,822	$ 80	1,669		$ 0	2,005	$ 62	825	5	5
Stockholders' equity, end of year at Dec. 31, 2015								$ 6,946	4,082					2,897
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(60)
Net income attributable to Ameren common shareholders	653			653
Net income	659								360				360	255				255
Common stock dividends				(416)									(355)					(110)
Capital contribution from parent									$ 44		6			0		0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ 20				(20)									$ 5						(5)
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2016	242.6								102.1					25.5
End of year at Dec. 31, 2016	$ 7,245	2	5,556	1,568	(23)	(23)	142			511	1,828	80	1,671		0	2,005	62	967	0	0
Stockholders' equity, end of year at Dec. 31, 2016	7,103							7,103	$ 4,090					$ 3,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(16)
Net income attributable to Ameren common shareholders	523			523
Net income	529								326				326	271				271
Common stock dividends				(431)									(362)					0
Capital contribution from parent									$ 30		30			8		8
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ (5)				5									$ 0						0
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2017	242.6								102.1					25.5
End of year at Dec. 31, 2017	$ 7,326	$ 2	$ 5,540	$ 1,660	$ (18)	$ (18)	$ 142			$ 511	$ 1,858	$ 80	$ 1,632		$ 0	$ 2,013	$ 62	$ 1,235	$ 0	$ 0
Stockholders' equity, end of year at Dec. 31, 2017	$ 7,184							$ 7,184	$ 4,081					$ 3,310