Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds		$ 211.1	$ 172.0
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold to third party		248.1
Fair value of deferred proceeds		211.1	172.0
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable		157.6	109.7
Unbilled utility revenues		90.4	71.3
Other receivables		0.1	0.1
Receivables sold to third party		248.1	181.1
Less: cash proceeds	[1]	21.0	5.0
Deferred proceeds		227.1	176.1
Less: allowance for doubtful accounts		16.0	4.1
Fair value of deferred proceeds		211.1	172.0
Outstanding receivables past due		$ 68.0	$ 18.0

[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.