Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	$ 111.7	$ 93.8
Unbilled utility revenues	90.2	83.1
Deferred proceeds	211.1	172.0
Other	89.0	53.5
Allowance for doubtful accounts	(8.7)	(4.8)
Accounts receivable, less allowance for doubtful accounts	493.3	397.6
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	0.0	4.6
Unbilled utility revenues	0.0	1.2
Deferred proceeds	211.1	172.0
Other	30.7	22.8
Allowance for doubtful accounts	(1.1)	(0.6)
Accounts receivable, less allowance for doubtful accounts	240.7	200.0
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	104.4	81.5
Unbilled utility revenues	90.2	81.9
Other	38.8	25.7
Allowance for doubtful accounts	(7.1)	(3.7)
Accounts receivable, less allowance for doubtful accounts	$ 226.3	$ 185.4