Common Equity (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Schedule of Common Equity [Line Items]
Shares available for issuance under the OIP, Shareowner Direct Plan and 401k Savings Plan	8,400,000
Ownership percentage	15.00%
Initial right entitlement, number of shares of common stock shareholders have the option to purchase (in shares)	0.5
Exercise price, rights (in dollars per share)	110
Percentage of exercise price upon change in beneficial ownership	200.00%
IPL [Member]
Schedule of Common Equity [Line Items]
Common equity ratio, computation of ratio average, number of months	13 months
Common equity ratio	42.00%
Retained earnings free of dividend restrictions	$ 387
WPL [Member]
Schedule of Common Equity [Line Items]
Common equity ratio, computation of ratio average, number of months	13 months
Common equity ratio	51.03%
Retained earnings free of dividend restrictions	$ 119
Minimum [Member]
Schedule of Common Equity [Line Items]
Ownership percentage	10.00%
Minimum [Member] | WPL [Member]
Schedule of Common Equity [Line Items]
Common equity ratio	25.00%