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			37
Stock-based compensation activity			(15)
Capital contribution from parent														$ 0		0
Net income	$ 373								$ 148				148	184				184
Net income attributable to Ameren common shareholders	370			370
Common stock dividends				(233)									(100)
Preferred stock dividends													(2)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (3)						3
Dividends paid to noncontrolling interest holders							(3)
Common stock shares outstanding at beginning of year at Dec. 31, 2018	244,500,000
Shares issued under the DRPlus and 401(k) plan	500,000
Shares issued for stock-based compensation	800,000
Common stock shares outstanding at end of period at Jun. 30, 2019	245,800,000
End of period at Jun. 30, 2019	$ 7,933	$ 2	5,649	2,161	(21)	$ (21)	142			$ 511	$ 1,903	$ 80	1,781		$ 0	2,173	$ 62	1,721
Dividends per common share (in dollars per share)	$ 0.9500
Shareholders' equity, end of year at Jun. 30, 2019								$ 7,791	4,275					3,956
Beginning of period at Mar. 31, 2019			5,625	2,099	(21)		142						1,774			2,173		1,659
Shares issued under the DRPlus and 401(k) plan			18
Stock-based compensation activity			6
Capital contribution from parent																0
Net income	$ 180								108				108	63				63
Net income attributable to Ameren common shareholders	179			179
Common stock dividends				(117)									(100)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holders	$ (1)						1
Dividends paid to noncontrolling interest holders							(1)
Common stock shares outstanding at beginning of year at Mar. 31, 2019	245,600,000
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 Jun. 30, 2019	245,800,000
End of period at Jun. 30, 2019	$ 7,933	2	5,649	2,161	(21)	(21)	142			511	1,903	80	1,781		0	2,173	62	1,721
Dividends per common share (in dollars per share)	$ 0.4750
Shareholders' equity, end of year at Jun. 30, 2019								7,791	4,275					3,956
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			27
Stock-based compensation activity			(5)
Capital contribution from parent														(350)		350
Net income	392								$ 144				144	$ 205				205
Net income attributable to Ameren common shareholders	389			389
Common stock dividends				(244)									0
Preferred stock dividends													(2)					(2)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (3)						3
Dividends paid to noncontrolling interest holders							(3)
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	400,000
Shares issued for stock-based compensation	500,000
Common stock shares outstanding at end of period at Jun. 30, 2020	247,100,000								102,100,000					25,500,000
End of period at Jun. 30, 2020	$ 8,369	2	5,716	2,525	(16)	(16)	142			511	2,027	80	1,873		0	2,538	62	2,085
Dividends per common share (in dollars per share)	$ 0.9900
Shareholders' equity, end of year at Jun. 30, 2020	$ 8,227							8,227	$ 4,491					$ 4,685
Beginning of period at Mar. 31, 2020			5,695	2,404	(16)								1,721			2,288		2,002
Shares issued under the DRPlus and 401(k) plan			14
Stock-based compensation activity			7
Capital contribution from parent																250
Net income	244								$ 153				153	$ 84				84
Net income attributable to Ameren common shareholders	243			243
Common stock dividends				(122)									0
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holders	$ (1)						1
Dividends paid to noncontrolling interest holders							(1)
Common stock shares outstanding at beginning of year at Mar. 31, 2020	246,900,000
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 Jun. 30, 2020	247,100,000								102,100,000					25,500,000
End of period at Jun. 30, 2020	$ 8,369	$ 2	$ 5,716	$ 2,525	$ (16)	$ (16)	$ 142			$ 511	$ 2,027	$ 80	$ 1,873		$ 0	$ 2,538	$ 62	$ 2,085
Dividends per common share (in dollars per share)	$ 0.4950
Shareholders' equity, end of year at Jun. 30, 2020	$ 8,227							$ 8,227	$ 4,491					$ 4,685