Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds		$ 172.0	$ 177.2
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold to third party		181.1
Fair value of deferred proceeds		172.0	177.2
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable		109.7	134.8
Unbilled utility revenues		71.3	69.7
Other receivables		0.1	0.1
Receivables sold to third party		181.1	204.6
Less: cash proceeds	[1]	5.0	22.0
Deferred proceeds		176.1	182.6
Less: allowance for doubtful accounts		4.1	5.4
Fair value of deferred proceeds		172.0	177.2
Outstanding receivables past due		$ 18.0	$ 19.9

[1] Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements.