Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Purchases, Sales, and Settlements	$ 4		$ 3
Fair value of plan assets	131		118		108
Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Purchases, Sales, and Settlements	(6)		(5)
Fair value of plan assets	15		19		23
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,461	[1]	3,127	[1]	2,814	[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	528		452
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,865		2,611
Pension Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	146		137
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44		29
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	5		1
Pension Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	39		28
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,269		1,090
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	107		83
Pension Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,162		1,007
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	273		235
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	273		235
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	515		435
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	143		134
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	372		301
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	860		832
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	860		832
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	149		176
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	149		176
Pension Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	256		250
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	256		250
Pension Benefits | Asset-Backed Securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27		17
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27		17
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	131		118
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	131		118
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15		19
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15		19
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1		0
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		0
Pension Benefits | Derivative liabilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(1)		(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)		(1)
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(112)	[2]	(102)	[2]
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34	[3]	29	[3]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,074	[1]	938	[1]	836	[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	490		439
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	511		443
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	77		83
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	77		83
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	398		333
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	297		245
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101		88
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	77		66
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	77		66
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	135		114
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	39		45
Postretirement Benefits | International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	96		69
Postretirement Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		88
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		88
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103		91
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103		91
Postretirement Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72		67
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	72		67
Postretirement Benefits | Asset-Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10		18
Postretirement Benefits | Asset-Backed Securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			18
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40		22
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40		22
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	112	[2]	102	[2]
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(39)	[4]	(46)	[4]
Includes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member] | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,074		938
Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,461		3,127
Excludes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member] | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,001		882
Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,539		$ 3,200

[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 Medicare, interest receivables, and receivables related to pending security sales.