Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Revenues:
Electric	$ 1,223	$ 1,207
Natural gas	362	308
Total operating revenues	1,585	1,515
Operating Expenses:
Fuel	188	206
Purchased power	163	180
Natural gas purchased for resale	171	130
Other operations and maintenance	431	418
Depreciation and amortization	234	221
Taxes other than income taxes	125	118
Total operating expenses	1,312	1,273
Operating Income	273	242
Other Income (Expense), Net	23	18
Interest Charges	101	99
Income Before Income Taxes	195	161
Income Taxes	42	57
Net income	153	104
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net Income Attributable to Ameren Common Shareholders	151	102
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	154	104
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income	$ 152	$ 102
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted (In dollars per share)	$ 0.62	$ 0.42
Dividends per Common Share (in dollars per share)	$ 0.4575	$ 0.44
Average Common Shares Outstanding - Basic (in shares)	242.9	242.6
Union Electric Company
Operating Revenues:
Electric	$ 741	$ 747
Natural gas	51	44
Total operating revenues	792	791
Operating Expenses:
Fuel	188	206
Purchased power	42	91
Natural gas purchased for resale	24	20
Other operations and maintenance	232	219
Depreciation and amortization	136	133
Taxes other than income taxes	80	75
Total operating expenses	702	744
Operating Income	90	47
Other Income (Expense), Net	13	16
Interest Charges	51	54
Income Before Income Taxes	52	9
Income Taxes	13	3
Net income	39	6
Preferred Stock Dividends	1	1
Net Income Available to Common Shareholder	38	5
Ameren Illinois Company
Operating Revenues:
Electric	449	439
Natural gas	311	264
Total operating revenues	760	703
Operating Expenses:
Purchased power	124	101
Natural gas purchased for resale	147	110
Other operations and maintenance	199	200
Depreciation and amortization	90	83
Taxes other than income taxes	41	40
Total operating expenses	601	534
Operating Income	159	169
Other Income (Expense), Net	6	0
Interest Charges	37	37
Income Before Income Taxes	128	132
Income Taxes	32	52
Net income	96	80
Preferred Stock Dividends	1	1
Net Income Available to Common Shareholder	$ 95	$ 79