Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Operating revenues	$ 750.9	$ 887.6	$ 690.3	$ 765.7	$ 760.8	$ 870.9	$ 712.5	$ 877.2	$ 3,094.5	$ 3,221.4	$ 3,262.1
Depreciation and amortization									332.4	321.0	287.3
Operating income (loss)	101.6	213.7	108.8	95.6	106.0	209.4	70.6	127.3	519.7	513.3	560.0
Interest expense, net of AFUDC									134.8	146.3	144.8
Equity income from unconsolidated investments, net									(41.3)	(39.3)	(38.1)
Interest income and other									(4.0)	(4.3)	(4.6)
Income tax expense (benefit)									89.4	69.2	147.7
Income from continuing operations, net of tax									340.8	341.4	310.2
Loss from discontinued operations, net of tax									(5.1)	(19.5)	(3.9)
Net income									335.7	321.9	306.3
Preferred dividends									15.9	18.3	18.7
Net income attributable to Alliant Energy common shareowners	68.3	150.7	65.9	34.9	57.0	122.0	51.1	73.5	319.8	303.6	287.6
Total assets	10,785.5				9,687.9				10,785.5	9,687.9	9,282.9
Investments in equity method subsidiaries	266.6				249.6				266.6	249.6	238.5
Construction and acquisition expenditures									1,158.1	673.4	866.9
IPL and WPL [Member]
Segment Reporting Information [Line Items]
Number of reportable segments (in segments)									3
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	393.2	497.7	360.7	398.7	404.0	484.5	374.1	477.5	1,650.3	1,740.1	1,795.8
Depreciation and amortization									188.9	179.1	177.0
Operating income (loss)	29.0	112.0	35.4	23.9	26.3	116.5	18.5	47.1	200.3	208.4	261.9
Interest income and other									(0.2)	(0.2)	(0.5)
Income tax expense (benefit)									(19.8)	(3.6)	42.3
Net income	25.6	106.5	19.6	(1.5)	9.8	101.4	1.0	27.1	150.2	139.3	143.4
Preferred dividends									12.6	15.0	15.4
Net income attributable to Alliant Energy common shareowners	22.4	103.3	16.6	(4.7)	6.6	98.3	(2.3)	21.7	137.6	124.3	128.0
Total assets	5,457.0				5,093.5				5,457.0	5,093.5
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	345.2	376.6	315.7	354.5	344.5	374.6	326.8	388.5	1,392.0	1,434.4	1,423.6
Depreciation and amortization									140.9	140.1	108.6
Operating income (loss)	65.3	93.4	61.5	64.6	73.2	86.9	45.5	74.8	284.8	280.4	278.8
Equity income from unconsolidated investments, net									(42.1)	(38.7)	(37.8)
Interest income and other									(0.1)	0	(0.1)
Income tax expense (benefit)									94.6	81.9	98.3
Net income	41.0	56.7	36.1	31.9	42.7	51.4	25.0	44.4	165.7	163.5	152.3
Preferred dividends									3.3	3.3	3.3
Net income attributable to Alliant Energy common shareowners	40.2	55.9	35.2	31.1	41.9	50.6	24.1	43.6	162.4	160.2	149.0
Total assets	4,762.6				4,044.0				4,762.6	4,044.0
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,589.3	2,635.8	2,674.2
Depreciation and amortization									299.3	289.0	255.1
Operating income (loss)									426.2	444.2	489.8
Equity income from unconsolidated investments, net									(42.1)	(38.7)	(37.8)
Total assets	8,438.8				7,524.5				8,438.8	7,524.5	7,227.2
Investments in equity method subsidiaries	264.3				246.5				264.3	246.5	236.0
Construction and acquisition expenditures									994.0	542.7	729.1
Electric [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,371.1	1,408.3	1,464.3
Depreciation and amortization									171.2	161.3	157.1
Operating income (loss)									166.2	181.6	239.7
Total assets	4,500.9				4,208.2				4,500.9	4,208.2	4,025.3
Construction and acquisition expenditures									291.0	245.6	298.8
Electric [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,218.2	1,227.5	1,209.9
Depreciation and amortization									128.1	127.7	98.0
Operating income (loss)									260.0	262.6	250.1
Equity income from unconsolidated investments, net									(42.1)	(38.7)	(37.8)
Total assets	3,937.9				3,316.3				3,937.9	3,316.3	3,201.9
Investments in equity method subsidiaries	264.3				246.5				264.3	246.5	236.0
Construction and acquisition expenditures									703.0	297.1	430.3
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									396.3	476.7	480.6
Depreciation and amortization									29.1	28.4	25.2
Operating income (loss)									51.5	47.8	53.4
Equity income from unconsolidated investments, net									0	0	0
Total assets	814.8				831.9				814.8	831.9	817.6
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									31.4	38.0	39.9
Gas [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									226.7	276.3	274.3
Depreciation and amortization									16.3	16.0	14.6
Operating income (loss)									24.2	20.6	23.6
Total assets	479.5				471.1				479.5	471.1	460.3
Construction and acquisition expenditures									16.4	21.2	20.0
Gas [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									169.6	200.4	206.3
Depreciation and amortization									12.8	12.4	10.6
Operating income (loss)									27.3	27.2	29.8
Equity income from unconsolidated investments, net									0	0	0
Total assets	335.3				360.8				335.3	360.8	357.3
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									15.0	16.8	19.9
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									56.7	62.0	64.6
Depreciation and amortization									1.4	1.8	5.3
Operating income (loss)									7.4	(3.2)	(2.5)
Equity income from unconsolidated investments, net									0	0	0
Total assets	966.0				781.1				966.0	781.1	782.4
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									0.1	27.4	64.3
Other Utility [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									52.5	55.5	57.2
Depreciation and amortization									1.4	1.8	5.3
Operating income (loss)									9.9	6.2	(1.4)
Total assets	476.6				414.2				476.6	414.2	452.0
Construction and acquisition expenditures									0.1	26.9	64.0
Other Utility [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									4.2	6.5	7.4
Depreciation and amortization									0	0	0
Operating income (loss)									(2.5)	(9.4)	(1.1)
Equity income from unconsolidated investments, net									0	0	0
Total assets	489.4				366.9				489.4	366.9	330.4
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									0	0.5	0.3
Total [Member]
Segment Reporting Information [Line Items]
Operating revenues									3,042.3	3,174.5	3,219.4
Depreciation and amortization									329.8	319.2	285.6
Operating income (loss)									485.1	488.8	540.7
Interest expense, net of AFUDC									136.8	146.6	142.8
Equity income from unconsolidated investments, net									(42.1)	(38.7)	(37.8)
Interest income and other									0.3	0.2	(0.6)
Income tax expense (benefit)									74.8	78.3	140.6
Income from continuing operations, net of tax									315.9	302.8	295.7
Loss from discontinued operations, net of tax									0	0	0
Net income									315.9	302.8	295.7
Preferred dividends									15.9	18.3	18.7
Net income attributable to Alliant Energy common shareowners									300.0	284.5	277.0
Total assets	10,219.6				9,137.5				10,219.6	9,137.5	8,827.2
Investments in equity method subsidiaries	264.3				246.5				264.3	246.5	236.0
Construction and acquisition expenditures									1,025.5	608.1	833.3
Total [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,650.3	1,740.1	1,795.8
Depreciation and amortization									188.9	179.1	177.0
Operating income (loss)									200.3	208.4	261.9
Interest expense, net of AFUDC									70.1	72.9	76.7
Interest income and other									(0.2)	(0.2)	(0.5)
Income tax expense (benefit)									(19.8)	(3.6)	42.3
Net income									150.2	139.3	143.4
Preferred dividends									12.6	15.0	15.4
Net income attributable to Alliant Energy common shareowners									137.6	124.3	128.0
Total assets	5,457.0				5,093.5				5,457.0	5,093.5	4,937.6
Construction and acquisition expenditures									307.5	293.7	382.8
Total [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,392.0	1,434.4	1,423.6
Depreciation and amortization									140.9	140.1	108.6
Operating income (loss)									284.8	280.4	278.8
Interest expense, net of AFUDC									66.7	73.7	66.1
Equity income from unconsolidated investments, net									(42.1)	(38.7)	(37.8)
Interest income and other									(0.1)		(0.1)
Income tax expense (benefit)									94.6	81.9	98.3
Net income									165.7	163.5	152.3
Preferred dividends									3.3	3.3	3.3
Net income attributable to Alliant Energy common shareowners									162.4	160.2	149.0
Total assets	4,762.6				4,044.0				4,762.6	4,044.0	3,889.6
Investments in equity method subsidiaries	264.3				246.5				264.3	246.5	236.0
Construction and acquisition expenditures									718.0	314.4	450.5
Non-Regulated [Member]
Segment Reporting Information [Line Items]
Operating revenues									52.2	46.9	42.7
Depreciation and amortization									2.6	1.8	1.7
Operating income (loss)									34.6	24.5	19.3
Interest expense, net of AFUDC									(2.0)	(0.3)	2.0
Equity income from unconsolidated investments, net									0.8	(0.6)	(0.3)
Interest income and other									3.7	4.1	(4.0)
Income tax expense (benefit)									14.6	(9.1)	7.1
Income from continuing operations, net of tax									24.9	38.6	14.5
Loss from discontinued operations, net of tax									(5.1)	(19.5)	(3.9)
Net income									19.8	19.1	10.6
Preferred dividends									0	0	0
Net income attributable to Alliant Energy common shareowners									19.8	19.1	10.6
Total assets	565.9				550.4				565.9	550.4	455.7
Investments in equity method subsidiaries	2.3				3.1				2.3	3.1	2.5
Construction and acquisition expenditures									$ 132.6	$ 65.3	$ 33.6

INTERSTATE POWER & LIGHT CO 10-K