Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 177.2		$ 203.5
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold to third party	204.6
Fair value of deferred proceeds	177.2		203.5
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	134.8		151.6
Unbilled utility revenues	69.7		86.2
Other receivables	0.1		0.2
Receivables sold to third party	204.6		238.0
Less: cash proceeds	22.0	[1]	29.0	[1]
Deferred proceeds	182.6		209.0
Less: allowance for doubtful accounts	5.4		5.5
Fair value of deferred proceeds	177.2		203.5
Outstanding receivables past due	$ 19.9		$ 21.5

[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.