Consolidated Statement of Comprehensive Income (Loss) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net income (loss)	$ 210	$ 139	$ (193)	$ 213
Other Comprehensive Income (Loss), Net of Taxes:
Unrealized net gain (loss) on derivative hedging instruments,net of income taxes (benefit)	2	(8)	14	(6)
Reclassification adjustments for derivative (gains) losses included in net income, net of income taxes (benefit)	2	7	4	3
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1		2	(1)
Other Comprehensive Income	5	(1)	20	(4)
Comprehensive Income (Loss)	215	138	(173)	209
Less: Comprehensive Income (Loss) Attributable to Noncontrolling Interest	(1)	1	(1)	4
Comprehensive Income (Loss)	216	137	(172)	205
Union Electric Company [Member]
Net income (loss)	144	91	166	113
Other Comprehensive Income (Loss), Net of Taxes:
Comprehensive Income (Loss)	144	91	166	113
Ameren Illinois Company [Member]
Net income (loss)	33	38	61	72
Other Comprehensive Income (Loss), Net of Taxes:
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	(1)	(1)	(2)	(2)
Comprehensive Income (Loss)	32	37	59	70
Ameren Energy Generating Company [Member]
Net income (loss)	(6)	13	(9)	35
Other Comprehensive Income (Loss), Net of Taxes:
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	4		5	1
Other Comprehensive Income	4		5	1
Comprehensive Income (Loss)	(2)	13	(4)	36
Less: Comprehensive Income (Loss) Attributable to Noncontrolling Interest	(2)		(4)	1
Comprehensive Income (Loss)		$ 13		$ 35