Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended			6 Months Ended			6 Months Ended			12 Months Ended	6 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended			1 Months Ended			1 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 WPL [Member]	Jun. 30, 2013 WPL [Member]	Dec. 31, 2013 WPL [Member]	Jun. 30, 2014 IPL [Member]	Jun. 30, 2013 IPL [Member]	Dec. 31, 2013 IPL [Member]	Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member]	Jun. 30, 2014 Alliant Energy and IPL [Member] Tax benefit riders [Member]	Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] Revenue Requirement Adjustment [Member]	Jul. 31, 2014 2015/2016 Test Period Retail Electric And Gas [Member] Subsequent Event [Member] WPL [Member]	Jun. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member]	Jun. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member]	Dec. 31, 2016 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member]	Dec. 31, 2015 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member]	Dec. 31, 2014 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member]	Jun. 30, 2014 2015 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	Dec. 31, 2015 2015 Test Year Retail Electric Fuel Related Request [Member] Scenario, Forecast [Member] WPL [Member]	Dec. 31, 2014 2015 Test Year Retail Electric Fuel Related Request [Member] Scenario, Forecast [Member] WPL [Member]	Dec. 31, 2013 2014 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	Jun. 30, 2014 2014 Test Year Retail Electric Fuel Related Request [Member] Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	$ 863.5		$ 821.5	$ 254.9		$ 206.6	$ 608.6		$ 614.9		$ 26.0
Authorized increase (decrease) in final rates, amount													(5)									19
Customer billing credits														20	20	10	25	70
Increase (decrease) in regulatory liabilities	45.2	(15.1)		47.9	12.0		(2.7)	(27.1)		(24.0)	(48.0)	(15.0)
Requested increase (decrease) in rates, amount																			55
Requested increase (decrease) in rates, percent																			5.00%
Under-collection of fuel-related costs outside of approved range																				$ 41	$ 14		$ 17
Authorized increase (decrease) in final rates, percent																						2.00%
Annual bandwidth for fuel-related costs																						2.00%