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, 2018			$ 5,627	$ 2,024	$ (22)		$ 142						$ 1,735			$ 2,173		$ 1,539
Shares issued under the DRPlus and 401(k) plan			19
Stock-based compensation activity			(21)
Capital contribution from parent														$ 0		0
Net income	$ 193								$ 40				40	121				121
Net income attributable to Ameren common shareholders	191			191
Common stock dividends				(116)
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)
Common stock shares outstanding at beginning of year at Dec. 31, 2018	244,500,000
Shares issued under the DRPlus and 401(k) plan	300,000
Shares issued for stock-based compensation	800,000
Common stock shares outstanding at end of period at Mar. 31, 2019	245,600,000
End of period at Mar. 31, 2019	$ 7,847	$ 2	5,625	2,099	(21)	$ (21)	142			$ 511	$ 1,903	$ 80	1,774		$ 0	2,173	$ 62	1,659
Dividends per common share (in dollars per share)	$ 0.4750
Shareholders' equity, end of year at Mar. 31, 2019								$ 7,705	4,268					3,894
Beginning of year 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			13
Stock-based compensation activity			(12)
Capital contribution from parent														(100)		100
Net income	148								$ (9)				(9)	$ 121				121
Net income attributable to Ameren common shareholders	146			146
Common stock dividends				(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)
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	200,000
Shares issued for stock-based compensation	500,000
Common stock shares outstanding at end of period at Mar. 31, 2020	246,900,000								102,100,000					25,500,000
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
Dividends per common share (in dollars per share)	$ 0.4950
Shareholders' equity, end of year at Mar. 31, 2020	$ 8,085							$ 8,085	$ 4,339					$ 4,352