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, 2019			$ 5,694	$ 2,380	$ (17)		$ 142				$ 2,027		$ 1,731			$ 2,188	$ 62	$ 1,882
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the ATM program			0
Shares issued under the DRPlus and 401(k) plan			27
Stock-based compensation activity			(5)
Capital contributions from parent									$ 0		0			$ (350)		350
Net income	$ 392								144				144	205				205
Net income attributable to Ameren common shareholders	389			389
Dividends on common stock				(244)
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)
Redemptions of preferred stock							0										0
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2019	246,200,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Shares issued under the DRPlus and 401(k) plan (in shares)	0
Shares issued under the DRPlus and 401(k) plan	400,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Jun. 30, 2020	247,100,000
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
Shareholders' equity, end of year at Jun. 30, 2020								$ 8,227	4,491					4,685
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.990
Beginning of period at Mar. 31, 2020			5,695	2,404	(16)		142				2,027		1,721			2,288		2,002
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the ATM program			0
Shares issued under the DRPlus and 401(k) plan			14
Stock-based compensation activity			7
Capital contributions from parent											0					250
Net income	$ 244								153				153	84				84
Net income attributable to Ameren common shareholders	243			243
Dividends on common stock				(122)
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)
Redemptions of preferred stock							0										0
Common stock shares outstanding at beginning of year (in shares) at Mar. 31, 2020	246,900,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Shares issued under the DRPlus and 401(k) plan (in shares)	0
Shares issued under the DRPlus and 401(k) plan	200,000
Shares issued for stock-based compensation (in shares)	0
Common stock shares outstanding at end of period (in shares) at Jun. 30, 2020	247,100,000
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
Shareholders' equity, end of year at Jun. 30, 2020								8,227	$ 4,491					$ 4,685
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.495
Beginning of period at Dec. 31, 2020	$ 9,080		6,179	2,757	(1)		142				2,518		2,101			2,652	62	2,252
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2020	253,300,000								102,100,000					25,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Mar. 31, 2021	255,500,000
End of period at Mar. 31, 2021			6,295	2,850	0		129				2,631		2,148			2,692		2,401
Beginning of period at Dec. 31, 2020	$ 9,080		6,179	2,757	(1)		142				2,518		2,101			2,652	62	2,252
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			113
Shares issued under the ATM program	121		121
Shares issued under the DRPlus and 401(k) plan			24
Stock-based compensation activity			(1)
Capital contributions from parent									$ (183)		183			$ (70)		70
Net income	443								$ 160				160	$ 236				236
Net income attributable to Ameren common shareholders	440			440
Dividends on common stock				(282)
Preferred stock dividends													(2)					(1)
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holders	$ (3)						3
Dividends paid to noncontrolling interest holders							(3)
Redemptions of preferred stock							(13)										(13)
Common stock shares outstanding at beginning of year (in shares) at Dec. 31, 2020	253,300,000								102,100,000					25,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	1,600,000
Shares issued under the DRPlus and 401(k) plan (in shares)	1,400,000
Shares issued under the DRPlus and 401(k) plan	300,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Jun. 30, 2021	257,100,000								102,100,000					25,500,000
End of period at Jun. 30, 2021	$ 9,482	3	6,436	2,915	(1)	(1)	129			511	2,701	80	2,259		0	2,722	49	2,487
Shareholders' equity, end of year at Jun. 30, 2021	$ 9,353							9,353	$ 5,551					$ 5,258
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 1.100
Beginning of period at Mar. 31, 2021			6,295	2,850	0		129				2,631		2,148			2,692		2,401
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Settlement of forward sale agreement through common shares issuance			0
Shares issued under the ATM program			121
Shares issued under the DRPlus and 401(k) plan			12
Stock-based compensation activity			8
Capital contributions from parent											70					30
Net income	$ 208								$ 112				112	$ 86				86
Net income attributable to Ameren common shareholders	207			207
Dividends on common stock				(142)
Preferred stock dividends													(1)					0
Change in deferred retirement benefit costs	(1)				(1)
Net income attributable to noncontrolling interest holders	$ (1)						1
Dividends paid to noncontrolling interest holders							(1)
Redemptions of preferred stock							0										0
Common stock shares outstanding at beginning of year (in shares) at Mar. 31, 2021	255,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Shares issued under the DRPlus and 401(k) plan (in shares)	1,400,000
Shares issued under the DRPlus and 401(k) plan	200,000
Shares issued for stock-based compensation (in shares)	0
Common stock shares outstanding at end of period (in shares) at Jun. 30, 2021	257,100,000								102,100,000					25,500,000
End of period at Jun. 30, 2021	$ 9,482	$ 3	$ 6,436	$ 2,915	$ (1)	$ (1)	$ 129			$ 511	$ 2,701	$ 80	$ 2,259		$ 0	$ 2,722	$ 49	$ 2,487
Shareholders' equity, end of year at Jun. 30, 2021	$ 9,353							$ 9,353	$ 5,551					$ 5,258
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.550