Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended			9 Months Ended	1 Months Ended		3 Months Ended		9 Months Ended			1 Months Ended			3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012 2012 Test Year Retail Electric Fuel Related, Request [Member]	Sep. 30, 2013 WPL [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2013 WPL [Member]	Sep. 30, 2012 WPL [Member]	Jul. 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas, Request [Member]	Jul. 31, 2013 WPL [Member] 2014 Test Year Retail Electric Fuel Related, Request [Member]	Dec. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Final Order [Member]	Sep. 30, 2013 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member]	Sep. 30, 2013 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Request [Member]	Dec. 31, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Request [Member]	Sep. 30, 2013 IPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2013 IPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2013 IPL [Member] Gas tax benefit rider [Member]	Jun. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]	May 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Gas tax benefit rider [Member]	Nov. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Settlement [Member]	Sep. 30, 2013 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member]	Sep. 30, 2013 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member]	Dec. 31, 2013 Scenario, Forecast [Member] IPL [Member] Revenue Requirement Adjustment [Member]
Utility Rate Cases [Line Items]
Reduction in annual rates for its customers, amount										$ 13		$ 29										$ 36
Proposed gas tax benefit rider, term																						3 years
Increase in interim rates for its customers, amount																					9
Increase in interim rates for its customers, percentage																					3.00%
Increase in annual rates for its customers, amount											31												11
Increase in annual rates for its customers, percentage											3.00%												4.00%
Revenue requirement adjustment			74.9	(5.0)				6.6	(32.5)									68.3	27.5	(8.0)						24.0
Electric revenue	798.1	815.3	2,043.4	2,000.3		340.5	358.7	906.0	929.6							457.6	456.6	1,137.4	1,070.7					7.0	18.0
Reduction in annual rates for its customers, percentage												3.00%
Rate at which fuel costs will be monitored													2.00%
Over-collection of fuel-related costs					17
Collected outside the approved range for 2012															11
Refunds to customers, including interest														$ 12