Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		$ (63.0)	$ (67.8)	$ 10.8
Regulatory liabilities		681.0	737.7
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return		$ 44.0
Period for simple average of actual costs incurred during Test Year		2 years
Increase (decrease) in regulatory liabilities		$ (67.3)	(75.5)	(18.9)
Regulatory liabilities		430.8	489.2
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return		9.0
Regulatory liabilities		250.2	248.5
Electric [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		(64.0)	(72.0)	(85.0)
Electric [Member] | Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities	$ (68.0)
Gas [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		$ (12.0)	$ (12.0)	$ (12.0)
Gas [Member] | Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities	(7.0)
Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] | WPL [Member]
Regulatory Matters [Line Items]
Expenditure collection period after new rate implementation		10 years
2017/2018 Test Period Retail Electric [Member] | Scenario, Forecast [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount	(9.0)
2017/2018 Test Period Retail Gas [Member] | Scenario, Forecast [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount	$ (9.0)
2015/2016 Test Period Retail Electric And Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		$ 5.0
2015/2016 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		6.0
2013/2014 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		6.0
2016 Test Year Retail Electric Fuel Related [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		$ 7.0
Annual bandwidth for fuel-related costs		2.00%