Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Other Paid-in Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Total Ameren Corporation Shareholders' Equity	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-in Capital	Union Electric Company Preferred Stock	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	Ameren Illinois Company Retained Earnings
Beginning of period at Dec. 31, 2017			$ 5,540	$ 1,660	$ (18)		$ 142						$ 1,632			$ 2,013		$ 1,235
Shares issued under the DRPlus and 401(k) plan			56
Stock-based compensation activity			2
Capital contribution from parent														$ (80)		80
Net income	$ 752								$ 503				503	222				222
Net income attributable to Ameren common shareholders	747			747
Common stock dividends				(334)									(225)
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of period at Dec. 31, 2017	242,600,000
Stock Issued During Period, Shares, New Issues	900,000
Stock Issued During Period, Shares, Other	700,000
Common stock shares outstanding at end of period at Sep. 30, 2018	244,200,000
End of period at Sep. 30, 2018	$ 7,798	$ 2	5,598	2,073	(17)	$ (17)	142			$ 511	$ 1,858	$ 80	1,907		$ 0	2,093	$ 62	1,455
Dividends per Common Share (in dollars per share)	$ 1.3725
Shareholders' equity, end of year at Sep. 30, 2018								$ 7,656	4,356					3,610
Beginning of period at Jun. 30, 2018			5,576	1,827	(19)		142						1,788			2,093		1,392
Shares issued under the DRPlus and 401(k) plan			16
Stock-based compensation activity			6
Capital contribution from parent																0
Net income	$ 359								295				295	63				63
Net income attributable to Ameren common shareholders	357			357
Common stock dividends				(111)									(175)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	2				2
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of period at Jun. 30, 2018	244,000,000.0
Stock Issued During Period, Shares, New Issues	200,000
Stock Issued During Period, Shares, Other	0
Common stock shares outstanding at end of period at Sep. 30, 2018	244,200,000
End of period at Sep. 30, 2018	$ 7,798	2	5,598	2,073	(17)	(17)	142			511	1,858	80	1,907		0	2,093	62	1,455
Dividends per Common Share (in dollars per share)	$ 0.4575
Shareholders' equity, end of year at Sep. 30, 2018								7,656	4,356					3,610
Beginning of period at Dec. 31, 2018	$ 7,773		5,627	2,024	(22)		142						1,735			2,173		1,539
Shares issued under the DRPlus and 401(k) plan			54
Stock-based compensation activity			(8)
Capital contribution from parent														0		0
Net income	739								$ 449				449	$ 249				249
Net income attributable to Ameren common shareholders	734			734
Common stock dividends				(350)									(250)
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of period at Dec. 31, 2018	244,500,000								102,100,000					25,500,000
Stock Issued During Period, Shares, New Issues	700,000
Stock Issued During Period, Shares, Other	800,000
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000								102,100,000					25,500,000
End of period at Sep. 30, 2019	$ 8,204	2	5,673	2,408	(21)	(21)	142			511	1,903	80	1,931		0	2,173	62	1,786
Dividends per Common Share (in dollars per share)	$ 1.4250
Shareholders' equity, end of year at Sep. 30, 2019	$ 8,062							8,062	$ 4,425					$ 4,021
Beginning of period at Jun. 30, 2019			5,649	2,161	(21)		142						1,781			2,173		1,721
Shares issued under the DRPlus and 401(k) plan			17
Stock-based compensation activity			7
Capital contribution from parent																0
Net income	366								$ 301				301	$ 65				65
Net income attributable to Ameren common shareholders	364			364
Common stock dividends				(117)									(150)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of period at Jun. 30, 2019	245,800,000
Stock Issued During Period, Shares, New Issues	200,000
Stock Issued During Period, Shares, Other	0
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000								102,100,000					25,500,000
End of period at Sep. 30, 2019	$ 8,204	$ 2	$ 5,673	$ 2,408	$ (21)	$ (21)	$ 142			$ 511	$ 1,903	$ 80	$ 1,931		$ 0	$ 2,173	$ 62	$ 1,786
Dividends per Common Share (in dollars per share)	$ 0.4750
Shareholders' equity, end of year at Sep. 30, 2019	$ 8,062							$ 8,062	$ 4,425					$ 4,021