Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds		$ 222.1	$ 211.1
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds		222.1	211.1
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable		133.8	157.6
Unbilled utility revenues		112.7	90.4
Other receivables		0.3	0.1
Receivables sold to third party		246.8	248.1
Less: cash proceeds	[1]	12.0	21.0
Deferred proceeds		234.8	227.1
Less: allowance for doubtful accounts		12.7	16.0
Fair value of deferred proceeds		222.1	211.1
Outstanding receivables past due		$ 44.7	$ 68.0

[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.