Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable [Line Items]
Customer	$ 91.6	$ 91.0
Unbilled utility revenues	82.1	74.2
Deferred proceeds	187.7	119.4
Other	48.0	76.3
Allowance for doubtful accounts	(7.3)	(10.5)
Accounts receivable, less allowance for doubtful accounts	402.1	350.4
IPL [Member]
Accounts Receivable [Line Items]
Customer	0.0	0.0
Unbilled utility revenues	0.0	0.0
Deferred proceeds	187.7	119.4
Other	16.3	37.2
Allowance for doubtful accounts	(1.2)	(3.1)
Accounts receivable, less allowance for doubtful accounts	202.8	153.5
WPL [Member]
Accounts Receivable [Line Items]
Customer	83.5	84.8
Unbilled utility revenues	82.1	74.2
Other	31.4	38.5
Allowance for doubtful accounts	(6.1)	(7.4)
Accounts receivable, less allowance for doubtful accounts	$ 190.9	$ 190.1