Consolidated Statement of Income (Loss) - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Operating Revenues:
Electric		$ 1,206	$ 1,102
Gas		308	332
Total operating revenues		1,514	1,434
Operating Expenses:
Fuel		206	203
Purchased power		180	138
Gas purchased for resale		130	152
Other operations and maintenance		405	400
Depreciation and amortization		221	207
Taxes other than income taxes		118	114
Total operating expenses		1,260	1,214
Operating Income		254	220
Other Income and Expenses:
Miscellaneous income	[1]	15	20
Miscellaneous expense	[1]	9	7
Total other income (expense)		6	13
Interest Charges		99	95
Income Before Income Taxes		161	138
Income Taxes		57	31
Net income		104	107
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(2)
Comprehensive Income		102	103
Less: Net Income Attributable to Noncontrolling Interests		2	2
Net Income Attributable to Ameren Common Shareholders		$ 102	$ 105
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted		$ 0.42	$ 0.43
Dividends per Common Share		$ 0.44	$ 0.425
Average Common Shares Outstanding - Basic		242.6	242.6
Union Electric Company
Operating Revenues:
Electric		$ 746	$ 694
Gas		44	47
Total operating revenues		790	741
Operating Expenses:
Fuel		206	203
Purchased power		91	42
Gas purchased for resale		20	21
Other operations and maintenance		212	212
Depreciation and amortization		133	127
Taxes other than income taxes		75	73
Total operating expenses		737	678
Operating Income		53	63
Other Income and Expenses:
Miscellaneous income		12	15
Miscellaneous expense		2	2
Total other income (expense)		10	13
Interest Charges		54	52
Income Before Income Taxes		9	24
Income Taxes		3	9
Net income		6	15
Other Comprehensive Income		0	0
Comprehensive Income		6	15
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1
Net Income Available to Common Stockholder		5	14
Ameren Illinois Company
Operating Revenues:
Electric		439	392
Gas		264	285
Total operating revenues		703	677
Operating Expenses:
Purchased power		101	104
Gas purchased for resale		110	131
Other operations and maintenance		197	194
Depreciation and amortization		83	77
Taxes other than income taxes		40	38
Total operating expenses		531	544
Operating Income		172	133
Other Income and Expenses:
Miscellaneous income		3	5
Miscellaneous expense		6	5
Total other income (expense)		(3)	0
Interest Charges		37	35
Income Before Income Taxes		132	98
Income Taxes		52	38
Net income		80	60
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)
Comprehensive Income		80	59
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1
Net Income Available to Common Stockholder		$ 79	$ 59

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.