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 year 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 DRPlus and 401(k) plan			13
Stock-based compensation activity			(12)
Capital contribution from parent									$ 0		0			$ (100)		100
Net income	$ 148								(9)				(9)	121				121
Net income attributable to Ameren common shareholders	146			146
Dividends on common stock				(122)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
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 the DRPlus and 401(k) plan (in shares)	200,000
Shares issued for stock-based compensation (in shares)	500,000
Common stock shares outstanding at end of period (in shares) at Mar. 31, 2020	246,900,000
End of period at Mar. 31, 2020	$ 8,227	$ 2	5,695	2,404	(16)	$ (16)	142			$ 511	2,027	$ 80	1,721		$ 0	2,288	62	2,002
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.495
Shareholders' equity, end of year at Mar. 31, 2020								$ 8,085	4,339					4,352
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	0
Beginning of year 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 DRPlus and 401(k) plan			12
Stock-based compensation activity			(9)
Capital contribution from parent									(113)		113			(40)		40
Net income	235								$ 48				48	$ 150				150
Net income attributable to Ameren common shareholders	233			233
Dividends on common stock				(140)
Preferred stock dividends													(1)					(1)
Change in deferred retirement benefit costs	1				1
Net income attributable to noncontrolling interest holders	$ (2)						2
Dividends paid to noncontrolling interest holders							(2)
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 the DRPlus and 401(k) plan (in shares)	100,000
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								102,100,000					25,500,000
End of period at Mar. 31, 2021	$ 9,277	$ 3	$ 6,295	$ 2,850	$ 0	$ 0	$ 129			$ 511	$ 2,631	$ 80	$ 2,148		$ 0	$ 2,692	$ 49	$ 2,401
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.550
Shareholders' equity, end of year at Mar. 31, 2021	$ 9,148							$ 9,148	$ 5,370					$ 5,142
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under forward sale agreement (in shares)	1,600,000