Long-Term Debt And Equity Financings (Schedule Of Outstanding Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 0.01
Preferred stock, authorized		100,000,000
Preferred stock, shares outstanding		0
Preferred stock, voluntary liquidation		$ 105.50
Union Electric Company and Ameren Illinois [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, issued		$ 142	$ 142
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 1
Preferred stock, authorized		7,500,000.0
Preferred stock, issued		$ 80	80
Union Electric Company | Par Value $100 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 100
Preferred stock, authorized		25,000,000
Union Electric Company | $3.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 3.50
Preferred stock, shares outstanding		130,000
Preferred stock, redemption price per share		$ 110.00
Preferred stock, issued		$ 13	13
Union Electric Company | $3.70 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 3.70
Preferred stock, shares outstanding		40,000
Preferred stock, redemption price per share		$ 104.75
Preferred stock, issued		$ 4	4
Union Electric Company | $4.00 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.00
Preferred stock, shares outstanding		150,000
Preferred stock, redemption price per share		$ 105.625
Preferred stock, issued		$ 15	15
Union Electric Company | $4.30 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.30
Preferred stock, shares outstanding		40,000
Preferred stock, redemption price per share		$ 105.00
Preferred stock, issued		$ 4	4
Union Electric Company | $4.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.50
Preferred stock, shares outstanding		213,595
Preferred stock, redemption price per share	[1]	$ 110.00
Preferred stock, issued		$ 21	21
Union Electric Company | $4.56 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.56
Preferred stock, shares outstanding		200,000
Preferred stock, redemption price per share		$ 102.47
Preferred stock, issued		$ 20	20
Union Electric Company | $4.75 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.75
Preferred stock, shares outstanding		20,000
Preferred stock, redemption price per share		$ 102.176
Preferred stock, issued		$ 2	2
Union Electric Company | $5.50 Series A
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 5.50
Preferred stock, shares outstanding		14,000
Preferred stock, redemption price per share		$ 110.00
Preferred stock, issued		$ 1	1
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized		2,600,000.0
Preferred stock, issued		$ 62	62
Ameren Illinois Company | Par Value $100 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 100
Preferred stock, authorized		2,000,000
Ameren Illinois Company | 4.00% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.00%
Preferred stock, shares outstanding		144,275
Preferred stock, redemption price per share		$ 101.00
Preferred stock, issued		$ 14	14
Ameren Illinois Company | 4.08% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.08%
Preferred stock, shares outstanding		45,224
Preferred stock, redemption price per share		$ 103.00
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 4.20% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.20%
Preferred stock, shares outstanding		23,655
Preferred stock, redemption price per share		$ 104.00
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.25% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.25%
Preferred stock, shares outstanding		50,000
Preferred stock, redemption price per share		$ 102.00
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 4.26% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.26%
Preferred stock, shares outstanding		16,621
Preferred stock, redemption price per share		$ 103.00
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.42% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.42%
Preferred stock, shares outstanding		16,190
Preferred stock, redemption price per share		$ 103.00
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.70% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.70%
Preferred stock, shares outstanding		18,429
Preferred stock, redemption price per share		$ 103.00
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.90% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.90%
Preferred stock, shares outstanding		73,825
Preferred stock, redemption price per share		$ 102.00
Preferred stock, issued		$ 7	7
Ameren Illinois Company | 4.92% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.92%
Preferred stock, shares outstanding		49,289
Preferred stock, redemption price per share		$ 103.50
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 5.16% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		5.16%
Preferred stock, shares outstanding		50,000
Preferred stock, redemption price per share		$ 102.00
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 6.625% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		6.625%
Preferred stock, shares outstanding		124,274
Preferred stock, redemption price per share		$ 100.00
Preferred stock, issued		$ 12	12
Ameren Illinois Company | 7.75% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		7.75%
Preferred stock, shares outstanding		4,542
Preferred stock, redemption price per share		$ 100.00
Preferred stock, issued		$ 1	$ 1

[1] In the event of voluntary liquidation, $105.50