Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended							12 Months Ended							12 Months Ended																12 Months Ended			12 Months Ended					8 Months Ended		12 Months Ended				12 Months Ended												12 Months Ended						12 Months Ended					12 Months Ended																					12 Months Ended							12 Months Ended		3 Months Ended		12 Months Ended																								1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009 MW	Dec. 31, 2012 Corporate Headquarters [Member]	Dec. 31, 2012 IPL's Tax Benefit Rider [Member]	Dec. 31, 2011 IPL's Tax Benefit Rider [Member]	Dec. 31, 2013 IPL's Tax Benefit Rider [Member] Subsequent Event [Member] Scenario, Forecast [Member]	Dec. 31, 2012 IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2011 IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2012 IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2011 IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2012 IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2011 IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2012 SO2 Emission Allowances [Member]		Dec. 31, 2012 Sheboygan Falls Energy Facility [Member] MW	Dec. 31, 2012 Franklin County Wind Project [Member] MW	Dec. 31, 2012 Vestas Wind Turbine Generators [Member] MW	Dec. 31, 2012 IPL [Member] customer	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Dec. 31, 2011 IPL [Member] Franklin County Wind Project [Member]	Dec. 31, 2012 IPL [Member] Ottumwa Unit 1 [Member]	Dec. 31, 2012 IPL [Member] George Neal Unit 3 [Member]	Dec. 31, 2012 IPL [Member] George Neal Unit 4 [Member]	Dec. 31, 2012 IPL [Member] IPL's Tax Benefit Rider [Member]	Dec. 31, 2011 IPL [Member] IPL's Tax Benefit Rider [Member]	Dec. 31, 2012 IPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2011 IPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2012 IPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2009 IPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2011 IPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2012 IPL [Member] Iowa [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2012 IPL [Member] IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2010 IPL [Member] IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2011 IPL [Member] IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2010 IPL [Member] IPL's Electric Transmission Service Costs [Member] Iowa [Member]	Feb. 28, 2010 IPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2007 IPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2012 IPL [Member] IPL Electric Transmission Assets Sale [Member] Iowa [Member]	Dec. 31, 2012 IPL [Member] Sutherland Four Cancellation Costs [Member]	Dec. 31, 2012 IPL [Member] Sutherland Four [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2010 IPL [Member] Sutherland Four [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2012 IPL [Member] Franklin County Iowa Wind Site [Member] MW	Dec. 31, 2007 IPL [Member] Franklin County Iowa Wind Site [Member] MW	Dec. 31, 2012 IPL [Member] SO2 Emission Allowances [Member]		Dec. 31, 2007 IPL [Member] SO2 Emission Allowances [Member]	Dec. 31, 2010 IPL [Member] Ipl S Sixth Street Costs [Member]	Dec. 31, 2012 IPL [Member] Franklin County Wind Project [Member] MW	Dec. 31, 2011 IPL [Member] Franklin County Wind Project [Member] MW	Dec. 31, 2010 IPL [Member] Sutherland Four Cancellation Costs [Member]	Dec. 31, 2009 IPL [Member] Sutherland Four [Member] MW	Dec. 31, 2012 IPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2010 IPL [Member] IPL Electric Transmission Assets Sale [Member] Iowa [Member]	Dec. 31, 2012 IPL [Member] Whispering Willow - East Wind Project [Member]	Dec. 31, 2009 IPL [Member] Whispering Willow - East Wind Project [Member]	Dec. 31, 2008 IPL [Member] Whispering Willow - East Wind Project [Member] IUB Cost Cap [Member] MW	Dec. 31, 2011 IPL [Member] Whispering Willow - East Wind Project [Member] Iowa [Member]	Dec. 31, 2012 IPL [Member] Whispering Willow - East Wind Project [Member] Minnesota [Member]	Dec. 31, 2011 IPL [Member] Whispering Willow - East Wind Project [Member] Retail Electric Rate [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2006 IPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2009 IPL [Member] IPL's Whispering Willow-East Wind Project [Member] MW	Dec. 31, 2009 IPL [Member] IPL's Whispering Willow-East Wind Project [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2012 IPL [Member] George Neal Unit 3 [Member]	Dec. 31, 2012 IPL [Member] George Neal Unit 4 [Member]	Dec. 31, 2012 IPL [Member] Generation [Member]	Dec. 31, 2011 IPL [Member] Generation [Member]	Dec. 31, 2010 IPL [Member] Generation [Member]	Dec. 31, 2012 IPL [Member] Distribution [Member]	Dec. 31, 2011 IPL [Member] Distribution [Member]	Dec. 31, 2010 IPL [Member] Distribution [Member]	Dec. 31, 2012 IPL [Member] Gas [Member]	Dec. 31, 2011 IPL [Member] Gas [Member]	Dec. 31, 2010 IPL [Member] Gas [Member]	Dec. 31, 2012 WPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Dec. 31, 2012 WPL [Member] Columbia Units 1-2 [Member]	Dec. 31, 2012 WPL [Member] IPL's Tax Benefit Rider [Member]	Dec. 31, 2011 WPL [Member] IPL's Tax Benefit Rider [Member]	Dec. 31, 2012 WPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2011 WPL [Member] IPL Electric Transmission Assets Sale [Member]	Dec. 31, 2012 WPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2011 WPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale [Member]	Dec. 31, 2012 WPL [Member] IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2011 WPL [Member] IPL's Electric Transmission Service Costs [Member]	Dec. 31, 2008 WPL [Member] Nelson Dewey Three [Member] MW	Dec. 31, 2012 WPL [Member] Edgewater Unit 5 and Columbia Units 1 and 2 [Member]	Dec. 31, 2011 WPL [Member] Edgewater Unit 5 and Columbia Units 1 and 2 [Member]	Dec. 31, 2011 WPL [Member] Bent Tree - Phase I Wind Project [Member]	Dec. 31, 2010 WPL [Member] Bent Tree - Phase I Wind Project [Member]	Dec. 31, 2012 WPL [Member] Nelson Dewey Three Cancellation Costs [Member]	Dec. 31, 2012 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member]	Dec. 31, 2012 WPL [Member] Riverside Energy Center [Member] MW	Dec. 31, 2012 WPL [Member] Riverside PPA [Member] MW	Jun. 30, 2011 WPL [Member] Green Lake and Fond du Lac Counties Wind Site [Member]	Dec. 31, 2012 WPL [Member] Columbia Units 1-2 [Member]	Dec. 31, 2012 WPL [Member] Generation [Member]	Dec. 31, 2011 WPL [Member] Generation [Member]	Dec. 31, 2010 WPL [Member] Generation [Member]	Dec. 31, 2012 WPL [Member] Generation [Member] Scenario, Forecast [Member]	Dec. 31, 2012 WPL [Member] Distribution [Member]	Dec. 31, 2011 WPL [Member] Distribution [Member]	Dec. 31, 2010 WPL [Member] Distribution [Member]	Dec. 31, 2012 WPL [Member] Distribution [Member] Scenario, Forecast [Member]	Dec. 31, 2012 WPL [Member] Gas [Member]	Dec. 31, 2011 WPL [Member] Gas [Member]	Dec. 31, 2010 WPL [Member] Gas [Member]	Dec. 31, 2012 WPL [Member] Gas [Member] Scenario, Forecast [Member]	Dec. 31, 2012 Alliant Energy And IPL [Member] Ottumwa Unit 1 [Member]	Dec. 31, 2012 Alliant Energy And IPL [Member] IPL's Tax Benefit Rider [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] IPL's Tax Benefit Rider [Member]	Dec. 31, 2012 Alliant Energy And IPL [Member] IPL's Tax Benefit Rider Decrease In Taxable Income [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] IPL's Tax Benefit Rider Decrease In Taxable Income [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] IPL Electric Transmission Assets Sale [Member] 2009 Test Year Retail Electric Request IA Final Order [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] IPL Electric Transmission Assets Sale [Member] 2009 Test Year Retail Electric Request MN [Member]	Dec. 31, 2010 Alliant Energy And IPL [Member] IPL' s Flood-Related Costs [Member]	Dec. 31, 2010 Alliant Energy And IPL [Member] Ipl S Sixth Street Costs [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] SO2 Emission Allowances [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] Cross State Air Pollution Rule [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] Cross State Air Pollution Rule [Member] Other Long-Term Liabilities And Deferred Credits [Member]	Nov. 30, 2011 Alliant Energy And IPL [Member] Sutherland Four [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] Sutherland Four [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member]	Dec. 31, 2012 Alliant Energy And IPL [Member] George Neal Units 3 And 4 [Member]	Dec. 31, 2009 Alliant Energy and WPL [Member] Nelson Dewey Three Cancellation Costs [Member]	Dec. 31, 2012 Alliant Energy and WPL [Member] WPL 2013/2014 Test Period Retail Electric And Gas Final Order [Member]	Dec. 31, 2012 Alliant Energy and WPL [Member] Edgewater Unit 5 Emission Controls [Member]	Dec. 31, 2012 Alliant Energy and WPL [Member] Columbia Units 1-2 [Member]	Dec. 31, 2012 IPL and WPL [Member] Whispering Willow - East Wind Project [Member] Bent Tree - Phase I Wind Project [Member] MW	Dec. 31, 2012 Franklin County Wind Project [Member]	Dec. 31, 2012 Sheboygan Falls Energy Facility [Member]	Dec. 31, 2012 Non-Regulated [Member] Minimum [Member]	Dec. 31, 2012 Non-Regulated [Member] Maximum [Member]	Dec. 31, 2013 Electric [Member] IPL's Tax Benefit Rider [Member] Scenario, Forecast [Member]	Dec. 31, 2013 Gas [Member] IPL's Tax Benefit Rider [Member] Scenario, Forecast [Member]
General:
Generation and distribution of steam, number of customers served (in customers)																				2
Fossil-fueled EGUs capacity (in megawatts)																	300																																					630																																			300
Captial lease, lease term	20 years
Capacity of electricity generation unit (in megawatts)																		100
Regulatory Assets and Regulatory Liabilities:
Regulatory assets representing past expenditures that did not earn a return																				$ 68,000,000																																																									$ 13,000,000
Simple average of actual costs incurred during test year, simple average basis																				2 years
Expenditure collection period after new rate implementation																																																																													5 years
Long-term purchase commitment, amount	982,000,000														34,000,000	[1]				537,000,000																											34,000,000	[1]	34,000,000																												303,000,000																																												34,000,000	34,000,000	34,000,000
Increase (decrease) in regulatory assets and liabilities																																																																																																																										30,000,000
Regulatory assets	1,612,400,000	1,495,000,000											16,600,000	24,900,000						1,217,900,000	1,117,300,000														16,600,000		24,900,000	41,000,000																																							394,500,000	377,700,000									0	0																															7,000,000
Amounts not probable of recovery																																																																																																																										4,000,000
Electric production fuel and energy purchases	712,300,000	764,500,000	819,200,000																	344,500,000	383,100,000	418,000,000																																																							367,800,000	381,400,000	401,200,000																																											2,000,000
Utility - other operation and maintenance	590,000,000	630,200,000	617,200,000																	350,000,000	375,000,000	384,500,000																																																							240,000,000	255,200,000	232,700,000																																							5,000,000	7,000,000	2,000,000		2,000,000
Amortization of regulatory asset																																			8,000,000							5,000,000																																																				2,000,000
Regulatory liabilities	916,100,000	910,100,000				355,800,000	349,600,000		32,500,000	45,100,000	9,500,000	14,600,000								701,400,000	721,300,000						355,800,000	349,600,000	32,500,000	45,100,000	9,500,000		14,600,000		41,000,000					89,000,000				26,000,000											46,000,000	46,000,000				23,000,000		3,000,000			29,000,000												214,700,000	188,800,000			0	0	0	0	0	0
Fossil-fueled EGUs capacity (in megawatts)																	300																																					630																																			300
Recovery of direct costs																																																					26,000,000																																																																							2,000,000				11,000,000
Recovery of dIrect costs, recovery period																																																																																																																												25 years				5 years
Amortization period of regulatory assets																																			5 years	5 years																	5 years
Increase in regulatory assets	178,100,000	413,100,000	20,800,000																	129,000,000	328,800,000	49,500,000																																																							49,100,000	84,300,000	(28,700,000)																																									16,000,000					2,000,000				5,000,000
Credit to utility other operation and maintenance																																																																																																																													2,000,000				5,000,000
Recognized impairment charges, regulatory assets		9,000,000																			2,000,000																																																									7,000,000
Jointly owned utility plant, proportionate ownership share																								48.00%	28.00%	25.70%																																					70.00%			28.00%	25.695%													46.20%															25.00%				46.20%
Allowance for funds used during construction	21,900,000	12,000,000	18,000,000																	8,400,000	5,800,000	5,500,000																												14,000,000																											13,500,000	6,200,000	12,500,000											11,000,000	3,000,000	1,000,000	10,000,000																			2,000,000															1,000,000			11,000,000	4,000,000
Increase (decrease) in regulatory liabilities	16,400,000	168,300,000	8,400,000					(24,000,000)												(12,100,000)	156,300,000	17,500,000									(5,000,000)	12,000,000		(8,000,000)																													59,000,000														28,500,000	12,000,000	(9,100,000)																																		(83,000,000)	(61,000,000)			19,000,000	5,000,000																			(56,000,000)	(12,000,000)
Income tax expense (benefit)	89,400,000	69,200,000	147,700,000																	(19,800,000)	(3,600,000)	42,300,000																																																							94,600,000	81,900,000	98,300,000																																		(48,000,000)	(36,000,000)	(35,000,000)	(25,000,000)
Reduction in electric customer rates																																							12,000,000
Amortization period of regulatory liabilities																																									5 years		5 years
Utility Property, Plant and Equipment:
Electric capacity of wind project (in megawatts)																		100	500																										200	500					100	100							200					200																																																																				400
Capitalized costs recorded in Other property, plant and equipment	403,500,000	0	0																																										13,000,000														417,000,000																		403,500,000	0	0																	403,500,000																72,000,000															66,000,000			132,000,000	126,000,000
Amount of electrical capacity (in megawatts)																																																																																																600	490
Asset impairment loss																																																										21,000,000																																								5,000,000																												8,000,000
Disallowed recovery of project costs																																																									8,000,000
Project costs allocated by jurisdiction																																																													30,000,000
Proceeds from sale of wind project assets to affiliate																				0	115,300,000	0	115,300,000
Purchase of development rights (in megawatts)				100
Jointly owned utility plant, proportionate ownership share																								48.00%	28.00%	25.70%																																					70.00%			28.00%	25.695%													46.20%															25.00%				46.20%
Allowance for funds used during construction	21,900,000	12,000,000	18,000,000																	8,400,000	5,800,000	5,500,000																												14,000,000																											13,500,000	6,200,000	12,500,000											11,000,000	3,000,000	1,000,000	10,000,000																			2,000,000															1,000,000			11,000,000	4,000,000
Intangible assets																																																																																																8,000,000
Intangible asset amortization expense, 2013																																																																																																5,400,000
Intangible asset amortization expense, 2014																																																																																																2,600,000
Average rate of depreciation for public utilities																																																																				3.70%	3.50%	3.70%	2.50%	2.40%	2.70%	3.40%	3.50%	3.30%																								3.20%	3.30%	2.90%	3.40%	2.90%	2.90%	2.60%	2.70%	2.60%	2.60%	2.20%	2.50%
Non-regulated and Other Property, Plant and Equipment:
Estimated useful life																																																																																																																																					30 years	35 years	5 years	30 years
Non-regulated generation property plant and equipment	258,600,000	270,600,000																																																																																																																																			148,000,000	111,000,000
Capitalized expenditures																																																																																																																																					130,000,000
Capitalized interest																																																																																																																																					9,000,000
Asset retirement obligation	101,500,000	91,100,000	75,900,000																	45,500,000	56,200,000	43,600,000																																																							46,900,000	34,900,000	32,300,000																																																						9,000,000
Increase in Alliant Energy Corporate Services, Inc. and other property, plant and equipment	132,600,000	65,300,000	33,600,000		48,000,000
Operating Revenues:
Estimated recovery of overaccrued operating revenue, reflected on customer billing, maximum period																				2 years
Final settlement recovery of overaccrued operating revenue, reflected on customer billing, maximum period																																																																													2 years
Utility Cost Recovery Mechanisms:
Contingent agreement, electric base rate case not to be filed, period			3 years
Other Comprehensive Income:
Other comprehensive income	$ 0	$ 600,000	$ 0

[1] Refer to Note 1(b) for discussion of $34 million of charges recognized by Alliant Energy and IPL in 2011 for forward contracts to purchase SO2 emission allowances.