Summary Of Significant Accounting Policies (Schedule Of Allowance For Doubtful Accounts) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	$ 4.0		$ 4.2
IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	0.7		0.9
WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	1.8		1.9
Customer [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	1.3	[1]	1.6	[1]
Customer [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	0	[1]	0	[1]
Customer [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	1.3	[1]	1.6	[1]
Other [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	2.7		2.6
Other [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	0.7		0.9
Other [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for doubtful accounts	$ 0.5		$ 0.3

[1] Refer to Note 4(a) for discussion of IPL’s sales of accounts receivable program.