Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Receivables [Line Items]
Customer	$ 100.8	$ 91.6
Unbilled utility revenues	68.9	82.1
Deferred proceeds	188.0	187.7
Other	47.7	48.0
Allowance for expected credit losses	(9.0)	(7.3)
Accounts receivable, less allowance for expected credit losses	396.4	402.1
IPL [Member]
Receivables [Line Items]
Customer	0.0	0.0
Unbilled utility revenues	0.0	0.0
Deferred proceeds	188.0	187.7
Other	17.8	16.3
Allowance for expected credit losses	(1.0)	(1.2)
Accounts receivable, less allowance for expected credit losses	204.8	202.8
WPL [Member]
Receivables [Line Items]
Customer	90.6	83.5
Unbilled utility revenues	68.9	82.1
Other	29.6	31.4
Allowance for expected credit losses	(8.0)	(6.1)
Accounts receivable, less allowance for expected credit losses	$ 181.1	$ 190.9