Consolidated Statement of Stockholders' Equity Consolidated Statement of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Other Paid-in Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric Company Common Stock	Union Electric Company Other Paid-in Capital	Union Electric Company Preferred Stock Not Subject To Mandatory Redemption	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 Not Subject To Mandatory Redemption	Ameren Illinois Company Retained Earnings
Beginning of year at Dec. 31, 2017			$ 5,540	$ 1,660	$ (18)		$ 142						$ 1,632			$ 2,013		$ 1,235
Stock Issued During Period, Value, New Issues			40
Stock-based compensation activity			(4)
Capital contribution from parent														$ 80		80
Net income	$ 393								$ 208				208	159				159
Net Income (Loss) Available to Common Stockholders, Basic	390			390
Common stock dividends				(223)									(50)
Preferred stock dividends													(2)					(2)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	1				(1)
Net income attributable to noncontrolling interest holder	$ 3						3
Dividends paid to noncontrolling interest holders							(3)
Beginning of year (shares) at Dec. 31, 2017	242.6
Stock Issued During Period, Shares, New Issues	0.7
Stock Issued During Period, Shares, Other	0.7
End of year (shares) at Jun. 30, 2018	244.0
End of year at Jun. 30, 2018	$ 7,528	$ 2	5,576	1,827	(19)	$ (19)	142			$ 511	$ 1,858	$ 80	1,788		$ 0	2,093	$ 62	1,392
Dividends per Common Share (in dollars per share)	$ 0.9150
Stockholders' equity, end of year at Jun. 30, 2018								$ 7,386	4,237					3,547
Beginning of year at Mar. 31, 2018			5,546	1,699	(17)		142						1,620			2,033		1,330
Stock Issued During Period, Value, New Issues			23
Stock-based compensation activity			7
Capital contribution from parent																60
Net income	$ 240								169				169	63				63
Net Income (Loss) Available to Common Stockholders, Basic	239			239
Common stock dividends				(112)									0
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				1
Change in deferred retirement benefit costs	2				(2)
Net income attributable to noncontrolling interest holder	$ 1						1
Dividends paid to noncontrolling interest holders							(1)
Beginning of year (shares) at Mar. 31, 2018	243.6
Stock Issued During Period, Shares, New Issues	0.4
Stock Issued During Period, Shares, Other	0.0
End of year (shares) at Jun. 30, 2018	244.0
End of year at Jun. 30, 2018	$ 7,528	2	5,576	1,827	(19)	(19)	142			511	1,858	80	1,788		0	2,093	62	1,392
Dividends per Common Share (in dollars per share)	$ 0.4575
Stockholders' equity, end of year at Jun. 30, 2018								7,386	4,237					3,547
Beginning of year at Dec. 31, 2018	$ 7,773		5,627	2,024	(22)		142						1,735			2,173		1,539
Stock Issued During Period, Value, New Issues			37
Stock-based compensation activity			(15)
Capital contribution from parent														0		0
Net income	373								$ 148				148	$ 184				184
Net Income (Loss) Available to Common Stockholders, Basic	370			370
Common stock dividends				(233)									(100)
Preferred stock dividends													(2)					(2)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	(1)				1
Net income attributable to noncontrolling interest holder	$ 3						3
Dividends paid to noncontrolling interest holders							(3)
Beginning of year (shares) at Dec. 31, 2018	244.5								102.1					25.5
Stock Issued During Period, Shares, New Issues	0.5
Stock Issued During Period, Shares, Other	0.8
End of year (shares) at Jun. 30, 2019	245.8								102.1					25.5
End of year 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
Stockholders' equity, end of year at Jun. 30, 2019	$ 7,791							7,791	$ 4,275					$ 3,956
Beginning of year at Mar. 31, 2019			5,625	2,099	(21)		142						1,774			2,173		1,659
Stock Issued During Period, Value, New Issues			18
Stock-based compensation activity			6
Capital contribution from parent																0
Net income	180								$ 108				108	$ 63				63
Net Income (Loss) Available to Common Stockholders, Basic	179			179
Common stock dividends				(117)									(100)
Preferred stock dividends													(1)					(1)
Stockholders' Equity, Other				0
Change in deferred retirement benefit costs	0				0
Net income attributable to noncontrolling interest holder	$ 1						1
Dividends paid to noncontrolling interest holders							(1)
Beginning of year (shares) at Mar. 31, 2019	245.6
Stock Issued During Period, Shares, New Issues	0.2
Stock Issued During Period, Shares, Other	0.0
End of year (shares) at Jun. 30, 2019	245.8								102.1					25.5
End of year 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
Stockholders' equity, end of year at Jun. 30, 2019	$ 7,791							$ 7,791	$ 4,275					$ 3,956