Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	$ 103.3	$ 111.7
Unbilled utility revenues	85.1	90.2
Deferred proceeds	222.1	211.1
Other	84.3	89.0
Allowance for doubtful accounts	(12.0)	(8.7)
Accounts receivable, less allowance for doubtful accounts	482.8	493.3
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	0.0	0.0
Unbilled utility revenues	0.0	0.0
Deferred proceeds	222.1	211.1
Other	44.1	30.7
Allowance for doubtful accounts	(1.3)	(1.1)
Accounts receivable, less allowance for doubtful accounts	264.9	240.7
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	97.7	104.4
Unbilled utility revenues	85.1	90.2
Other	40.1	38.8
Allowance for doubtful accounts	(10.7)	(7.1)
Accounts receivable, less allowance for doubtful accounts	$ 212.2	$ 226.3