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	Total Ameren Corporation Shareholders' Equity	Noncontrolling Interests	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, 2020			$ 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 contributions 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
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	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
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
Shareholders' equity, end of year at Mar. 31, 2021							$ 9,148		5,370					5,142
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.55
Beginning of year at Dec. 31, 2021	$ 9,829		6,502	3,182	13			129			2,725		2,595			2,914	49	2,677
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			(8)
Capital contributions from parent									0		0			0		0
Net income	253								$ 51				51	$ 169				169
Net income attributable to Ameren common shareholders	252			252
Dividends on common stock				(152)
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 Dec. 31, 2021	257,700,000								102,100,000					25,500,000
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	100,000
Shares issued for stock-based compensation (in shares)	400,000
Common stock shares outstanding at end of period (in shares) at Mar. 31, 2022	258,200,000								102,100,000					25,500,000
End of period at Mar. 31, 2022	$ 9,935	$ 3	$ 6,507	$ 3,282	$ 14	$ 14		$ 129		$ 511	$ 2,725	$ 80	$ 2,645		$ 0	$ 2,914	$ 49	$ 2,846
Shareholders' equity, end of year at Mar. 31, 2022	$ 9,806						$ 9,806		$ 5,961					$ 5,809
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.59