Consolidated Statement of Comprehensive Income (Loss) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net income (Loss)	$ (403)	$ 74
Other Comprehensive Income (Loss), Net of Taxes [Abstract]
Unrealized net gain on derivative hedging instruments, net of income taxes	12	2
Reclassification adjustments for derivative (gains) losses included in net income, net of income taxes (benefit)	2	(4)
Pension and other postretirement activity, net of income taxes	1	(1)
Total other comprehensive income (loss), net of taxes	15	(3)
Comprehensive Income (Loss)	(388)	71
Less: Comprehensive Income Attributable to Noncontrolling Interests		3
Comprehensive Income (Loss)	(388)	68
Union Electric Company [Member]
Net income (Loss)	22	22
Other Comprehensive Income (Loss), Net of Taxes [Abstract]
Comprehensive Income (Loss)	22	22
Ameren Illinois Company [Member]
Net income (Loss)	28	34
Other Comprehensive Income (Loss), Net of Taxes [Abstract]
Pension and other postretirement activity, net of income taxes	(1)	(1)
Comprehensive Income (Loss)	27	33
Ameren Energy Generating Company [Member]
Net income (Loss)	(3)	22
Other Comprehensive Income (Loss), Net of Taxes [Abstract]
Pension and other postretirement activity, net of income taxes	1	1
Total other comprehensive income (loss), net of taxes	1	1
Comprehensive Income (Loss)	(2)	23
Comprehensive Income (Loss)	$ (2)	$ 23