Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable [Line Items]
Customer	$ 91.0	$ 103.3
Unbilled utility revenues	74.2	85.1
Deferred proceeds	119.4	222.1
Other	76.3	84.3
Allowance for doubtful accounts	(10.5)	(12.0)
Accounts receivable, less allowance for doubtful accounts	350.4	482.8
IPL [Member]
Accounts Receivable [Line Items]
Customer	0.0	0.0
Unbilled utility revenues	0.0	0.0
Deferred proceeds	119.4	222.1
Other	37.2	44.1
Allowance for doubtful accounts	(3.1)	(1.3)
Accounts receivable, less allowance for doubtful accounts	153.5	264.9
WPL [Member]
Accounts Receivable [Line Items]
Customer	84.8	97.7
Unbilled utility revenues	74.2	85.1
Other	38.5	40.1
Allowance for doubtful accounts	(7.4)	(10.7)
Accounts receivable, less allowance for doubtful accounts	$ 190.1	$ 212.2