Receivables (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable and unbilled revenues	$ 188.8	$ 120.5
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold	195.3	219.6
Customer accounts receivable and unbilled revenues	75.2	174.1
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable and unbilled revenues	76.2	84.2
Maximum [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party	160.0	160.0	170.0
Maximum [Member] | March 2011 Through March 2012 [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party	160.0
RMT [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable and unbilled revenues	101.0	26.0
WPC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable and unbilled revenues	$ 27.0	$ 26.0
Obligations to pay on tax benefits, percentage	90.00%
Advances For Customer Energy Efficiency Projects [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable term, years	5