Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to net periodic benefit costs, percentage		40.00%
Income statement impact of adoption of Accounting Standards Update 2017-17		$ 4.3
Cash receipts on sold receivables	$ 284.3	$ 214.7
IPL [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to net periodic benefit costs, percentage		40.00%
Income statement impact of adoption of Accounting Standards Update 2017-17		$ 1.7
Cash receipts on sold receivables	$ 284.3	$ 214.7
WPL [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to net periodic benefit costs, percentage		40.00%
Income statement impact of adoption of Accounting Standards Update 2017-17		$ 2.6
Alliant Energy and IPL [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash receipts on sold receivables		$ 214.7	$ 681.9