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, 2018			$ 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 year at Dec. 31, 2018	244,500,000
Shares issued under the DRPlus and 401(k) plan	700,000
Shares issued for stock-based compensation	800,000
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000.0
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	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 year at Jun. 30, 2019	245,800,000
Shares issued under the DRPlus and 401(k) plan	200,000
Shares issued for stock-based compensation	0
Common stock shares outstanding at end of period at Sep. 30, 2019	246,000,000.0
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	4,425					4,021
Beginning of period at Dec. 31, 2019	$ 8,201		5,694	2,380	(17)		142						1,731			2,188		1,882
Shares issued under the DRPlus and 401(k) plan			37
Stock-based compensation activity			2
Capital contribution from parent														(350)		350
Net income	761								$ 442				442	$ 282				282
Net income attributable to Ameren common shareholders	756			756
Common stock dividends				(367)									0
Preferred stock dividends													(3)					(2)
Change in deferred retirement benefit costs	2				2
Net income attributable to noncontrolling interest holders	$ (5)						5
Dividends paid to noncontrolling interest holders							(5)
Common stock shares outstanding at beginning of year at Dec. 31, 2019	246,200,000								102,100,000					25,500,000
Shares issued under the DRPlus and 401(k) plan	500,000
Shares issued for stock-based compensation	500,000
Common stock shares outstanding at end of period at Sep. 30, 2020	247,200,000								102,100,000					25,500,000
End of period at Sep. 30, 2020	$ 8,631	2	5,733	2,769	(15)	(15)	142			511	2,027	80	2,170		0	2,538	62	2,162
Dividends per common share (in dollars per share)	$ 1.4850
Shareholders' equity, end of year at Sep. 30, 2020	$ 8,489							8,489	$ 4,788					$ 4,762
Beginning of period at Jun. 30, 2020			5,716	2,525	(16)		142						1,873			2,538		2,085
Shares issued under the DRPlus and 401(k) plan			10
Stock-based compensation activity			7
Capital contribution from parent																0
Net income	369								$ 298				298	$ 77				77
Net income attributable to Ameren common shareholders	367			367
Common stock dividends				(123)									0
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
Common stock shares outstanding at beginning of year at Jun. 30, 2020	247,100,000
Shares issued under the DRPlus and 401(k) plan	100,000
Shares issued for stock-based compensation	0
Common stock shares outstanding at end of period at Sep. 30, 2020	247,200,000								102,100,000					25,500,000
End of period at Sep. 30, 2020	$ 8,631	$ 2	$ 5,733	$ 2,769	$ (15)	$ (15)	$ 142			$ 511	$ 2,027	$ 80	$ 2,170		$ 0	$ 2,538	$ 62	$ 2,162
Dividends per common share (in dollars per share)	$ 0.4950
Shareholders' equity, end of year at Sep. 30, 2020	$ 8,489							$ 8,489	$ 4,788					$ 4,762