Nuclear Decommissioning Trust Fund Investments (Tables)

12 Months Ended

Dec. 31, 2012

Nuclear Decommissioning Trust Fund Investments [Line Items]

Fair Value Of Securities In Nuclear Decommissioning Trust Fund

The following table presents the costs and fair values of investments in debt and equity securities in Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2012, and 2011:

Security Type	Cost			Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
2012
Debt securities	$	133		$	8	(a)		$	141
Equity securities	145			130		11		264
Cash	1			—		—		1
Other(b)	2			—		—		2
Total	$	281		$	138	$	11	$	408
2011
Debt securities	$	114		$	7	(a)		$	121
Equity securities	145			101		12		234
Cash	3			—		—		3
Other(b)	(1		)	—		—		(1		)
Total	$	261		$	108	$	12	$	357

(a) Amount less than $1 million.

(b) Represents payables relating to pending security purchases, net of receivables related to pending security sales and interest receivables.

Schedule Of Unrealized Losses On Available-For-Sale Investments Included In Nuclear Decommissioning Trust Fund

They are aggregated by investment category and the length of time that individual securities have been in a continuous unrealized loss position at December 31, 2012:

	Less than 12 Months				12 Months or Greater				Total
	Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses
Debt securities	$	17	$ (a)		$ (a)		$ (a)		$	17	$ (a)
Equity securities	7		1		14		10		21		11
Total	$	24	$	1	$	14	$	10	$	38	$	11

(a) Amount less than $1 million.

Regulatory Asset [Member]

Nuclear Decommissioning Trust Fund Investments [Line Items]

Proceeds From Sale Of Investments In Nuclear Decommissioning Trust Fund And Gross Realized Gains And Losses

The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2012, 2011, and 2010:

	2012		2011		2010
Proceeds from sales and maturities	$	384	$	199	$	256
Gross realized gains	6		5		5
Gross realized losses	2		4		4

Fair Value Of Securities In Nuclear Decommissioning Trust Fund According To Their Contractual Maturities

The following table presents the costs and fair values of investments in debt securities in Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2012:

	Cost		Fair Value
Less than 5 years	$	78	$	79
5 years to 10 years	32		35
Due after 10 years	23		27
Total	$	133	$	141