Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Receivables [Line Items]
Customer	$ 92.7	$ 91.6
Unbilled utility revenues	68.2	82.1
Deferred proceeds	194.1	187.7
Other	41.5	48.0
Allowance for expected credit losses	(10.0)	(7.3)
Accounts receivable, less allowance for expected credit losses	386.5	402.1
IPL [Member]
Receivables [Line Items]
Customer	0.0	0.0
Unbilled utility revenues	0.0	0.0
Deferred proceeds	194.1	187.7
Other	16.5	16.3
Allowance for expected credit losses	(1.7)	(1.2)
Accounts receivable, less allowance for expected credit losses	208.9	202.8
WPL [Member]
Receivables [Line Items]
Customer	82.4	83.5
Unbilled utility revenues	68.2	82.1
Other	24.5	31.4
Allowance for expected credit losses	(8.2)	(6.1)
Accounts receivable, less allowance for expected credit losses	$ 166.9	$ 190.9