Summary of Significant Accounting Policies (Long-Term Incentive Plan) (Details) (USD $)	6 Months Ended
	Jun. 30, 2011
Restricted Shares [Member]
Nonvested at January 1, 2011	83,154	[1]
Dividends	518	[1]
Forfeitures	(560)	[1]
Vested	(63,574)	[1],[2]
Nonvested at June 30, 2011	19,538	[1]
Nonvested at January 1, 2011	$ 49.87	[1]
Dividends	28.48	[1]
Forfeitures	$ 50.45	[1]
Vested	$ 49.47	[1],[2]
Nonvested at June 30, 2011	$ 51.21	[1]
Performance Share Units [Member]
Nonvested at January 1, 2011	1,142,768	[3]
Granted	731,962	[3],[4]
Forfeitures	(10,261)	[3]
Vested	(131,343)	[2],[3]
Nonvested at June 30, 2011	1,733,126	[3]
Nonvested at January 1, 2011	$ 23.96	[3]
Granted	$ 31.41	[3],[4]
Forfeitures	$ 26.14	[3]
Vested	$ 30.67	[2],[3]
Nonvested at June 30, 2011	$ 26.58	[3]

[1] Granted under the 1998 Plan.

[2] Shares/units vested due to Ameren attainment of performance goals and retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.

[3] Granted under the 2006 Plan.

[4] Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in January 2011 under the 2006 Plan.