Receivables (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds of receivables sold			$ 172.0	$ 177.2
Proceeds from Franklin County wind farm cash grant			0.0	0.0	$ 62.4
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party			46.7	46.4	105.9
Receivables sold to third party			181.1
Fair value of deferred proceeds of receivables sold			172.0	177.2
IPL [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party			137.0	150.0	170.0
Seasonal limit on cash proceeds to be received from third-party			150.0
Whiting Petroleum Corporation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Final payment of certain future tax benefits expected to be realized				26.0
Franklin County Wind Farm [Member] | American Recovery and Reinvestment Act of 2009 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from Franklin County wind farm cash grant					$ 62.4
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold to third party			181.1	204.6
Cash proceeds from receivables sold	[1]		5.0	22.0
Fair value of deferred proceeds of receivables sold			$ 172.0	$ 177.2
Scenario, Forecast [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party		$ 180.0

[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.