Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	$ 93.8	$ 88.2
Unbilled utility revenues	83.1	85.4
Deferred proceeds	172.0	177.2
Other	53.5	81.6
Allowance for doubtful accounts	(4.8)	(5.1)
Accounts receivable, less allowance for doubtful accounts	397.6	427.3
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	4.6	0.0
Unbilled utility revenues	1.2	0.0
Deferred proceeds	172.0	177.2
Other	22.8	39.9
Allowance for doubtful accounts	(0.6)	(0.4)
Accounts receivable, less allowance for doubtful accounts	200.0	216.7
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	81.5	81.0
Unbilled utility revenues	81.9	85.4
Other	25.7	23.6
Allowance for doubtful accounts	(3.7)	(4.2)
Accounts receivable, less allowance for doubtful accounts	$ 185.4	$ 185.8