Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Purchases, Sales, and Settlements	$ 5		$ 4
Fair value of plan assets	147		131		118
Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Purchases, Sales, and Settlements	(3)		(6)
Fair value of plan assets	13		15		19
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,794	[1]	3,461	[1]	3,127	[1]
Pension Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	411		528
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,327		2,865
Pension Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160		146
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38		44
Pension Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		5
Pension Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38		39
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,331		1,269
Pension Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		107
Pension Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,331		1,162
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270		273
Pension Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270		273
Pension Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		0
Pension Benefits | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	494		515
Pension Benefits | International and emerging markets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	134		143
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	360		372
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,026		860
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,026		860
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	175		149
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	175		149
Pension Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	372		256
Pension Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6
Pension Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	366		256
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31		27
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31		27
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147		131
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147		131
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13		15
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13		15
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1		1
Pension Benefits | Derivative assets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1		1
Pension Benefits | Derivative liabilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			(1)
Pension Benefits | Derivative liabilities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			(1)
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(125)	[2]	(112)	[2]
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	[3]	34	[3]
Pension Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,898		3,539
Pension Benefits | Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,794		3,461
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,109	[1]	1,074	[1]	938	[1]
Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	487		490
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	551		511
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		77
Postretirement Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		77
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		0
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	392		398
Postretirement Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	291		297
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101		101
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	70		77
Postretirement Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	70		77
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	131		135
Postretirement Benefits | International | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37		39
Postretirement Benefits | International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	94		96
Postretirement Benefits | Other Equity | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7		2
Postretirement Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	105		97
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	105		97
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	111		103
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	111		103
Postretirement Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		72
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		72
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44		40
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44		40
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	125	[2]	112	[2]
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(54)	[4]	(39)	[4]
Postretirement Benefits | Excludes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,038		1,001
Postretirement Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,109		$ 1,074

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.

[3] Receivables related to pending security sales, offset by payables related to pending security purchases.

[4] Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.