Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended						12 Months Ended	9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended			36 Months Ended	1 Months Ended					1 Months Ended
	Sep. 30, 2014 change	Sep. 30, 2013	Sep. 30, 2014 WPL [Member]	Sep. 30, 2013 WPL [Member]	Sep. 30, 2014 IPL [Member]	Sep. 30, 2013 IPL [Member]	Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member]	Sep. 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] WPL [Member]	Sep. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member]	Sep. 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]	Dec. 31, 2016 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, 2015 Fuel per MWh [Member] Scenario, Forecast [Member] WPL [Member]	Dec. 31, 2015 Increased sales volumes [Member] Scenario, Forecast [Member] WPL [Member]	Dec. 31, 2013 2014 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	Sep. 30, 2014 2014 Test Year Retail Electric Fuel Related Request [Member] Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Number of tax accounting method changes (in number of changes)	2
Increase (decrease) in regulatory assets and regulatory liabilities								$ 74
Authorized increase (decrease) in final rates, amount										(5)												19
Customer billing credits											26	46	10	25	70	105
Increase (decrease) in regulatory liabilities	61.1	(74.9)	46.3	(6.6)	14.8	(68.3)	(24.0)	(73.0)	(15.0)
Requested increase (decrease) in rates, amount																	55
Requested increase (decrease) in rates, percent																	5.00%
Fuel-related costs																		$ 41	$ 14	$ 28	$ 13		$ 21
Authorized increase (decrease) in final rates, percent																						2.00%
Annual bandwidth for fuel-related costs																						2.00%