Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				1 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended			1 Months Ended	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 WPL [Member]	Mar. 31, 2013 WPL [Member]	Dec. 31, 2013 WPL [Member] 2014 Test Year Retail Electric Fuel Related Request [Member]	Mar. 31, 2014 IPL [Member]	Mar. 31, 2013 IPL [Member]	Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member]	Mar. 31, 2014 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member]	Mar. 31, 2013 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member]	Mar. 31, 2014 Alliant Energy and WPL [Member] 2014 Test Year Retail Electric Fuel Related Request [Member]	Apr. 30, 2014 Subsequent Event [Member] WPL [Member] 2015/2016 Test Period Retail Electric And Gas [Member]	Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] Revenue Requirement Adjustment [Member]	Dec. 31, 2016 Scenario, Forecast [Member] IPL [Member] 2013 Test Year Retail Electric [Member]	Dec. 31, 2015 Scenario, Forecast [Member] IPL [Member] 2013 Test Year Retail Electric [Member]	Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] 2013 Test Year Retail Electric [Member]
Regulatory Matters [Line Items]
Requested rate decrease, amount												$ 5
Customer billing credits														10	25	70
Increase (decrease) in regulatory liabilities			26.1	5.8		(9.0)	14.7	(24.0)					(15.0)
Increase to electric revenues	675.8	633.2	301.6	283.0		374.2	350.2		3.9	5.5
Authorized increase (decrease) in final rates, amount					19
Authorized increase (decrease) in final rates, percent					2.00%
Annual bandwidth for fuel-related costs					2.00%
Under-collection of fuel-related costs outside of approved range											$ 10