Receivables (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 28, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds of receivables sold		$ 211.1	$ 172.0
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party		180.0	180.0	$ 180.0
Receivables sold to third party		248.1
Fair value of deferred proceeds of receivables sold		211.1	172.0
IPL [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash proceeds to be received from third-party		172.0	137.0	150.0
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
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold to third party		248.1	181.1
Cash proceeds from receivables sold	[1]	21.0	5.0
Fair value of deferred proceeds of receivables sold		$ 211.1	$ 172.0
Scenario, Forecast [Member] | IPL [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Limit on cash proceeds to be received from third-party					$ 125.0

[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.