Consolidated Statements Of Income (USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Utility:
Electric	$ 2,635.8	$ 2,674.2	$ 2,475.9
Gas	476.7	480.6	525.3
Other	62.0	64.6	92.9
Non-regulated	490.8	196.7	333.2
Total operating revenues	3,665.3	3,416.1	3,427.3
Utility:
Electric production fuel and energy purchases	764.5	819.2	891.4
Purchased electric capacity	257.2	279.7	281.1
Electric transmission service	323.8	279.5	225.4
Cost of gas sold	295.2	304.0	347.9
Other operation and maintenance	630.2	617.2	599.7
Non-regulated operation and maintenance	490.9	169.5	311.9
Depreciation and amortization	323.8	291.3	273.6
Taxes other than income taxes	101.3	99.6	100.1
Total operating expenses	3,186.9	2,860.0	3,031.1
Operating income	478.4	556.1	396.2
Interest expense and other:
Interest expense	158.3	163.0	154.9
Loss on early extinguishment of debt			203.0
Equity (income)/loss from unconsolidated investments, net	(39.3)	(38.1)	(36.6)
Allowance for funds used during construction	(12.0)	(18.0)	(39.7)
Interest income and other	(4.3)	(4.0)	(4.9)
Total interest expense and other	102.7	102.9	276.7
Income from continuing operations before income taxes	375.7	453.2	119.5
Income tax expense (benefit)	55.1	145.2	(9.3)
Income from continuing operations, net of tax	320.6	308.0	128.8
Income (loss) from discontinued operations, net of tax	1.3	(1.7)	0.9
Net income	321.9	306.3	129.7
Preferred dividend requirements of subsidiaries	18.3	18.7	18.7
Net income attributable to common shareowners	303.6	287.6	111.0
Weighted average number of common shares outstanding (basic) (000s)	110,626	110,442	110,268
Earnings per weighted average common share attributable to Alliant Energy common shareowners (basic):
Income from continuing operations, net of tax	$ 2.73	$ 2.62	$ 1.00
Income (loss) from discontinued operations, net of tax	$ 0.01	$ (0.02)	$ 0.01
Net income	$ 2.74	$ 2.60	$ 1.01
Weighted average number of common shares outstanding (diluted) (000s)	110,678	110,521	110,352
Earnings per weighted average common share attributable to Alliant Energy common shareowners (diluted):
Income from continuing operations, net of tax	$ 2.73	$ 2.62	$ 1.00
Income (loss) from discontinued operations, net of tax	$ 0.01	$ (0.02)	$ 0.01
Net income	$ 2.74	$ 2.60	$ 1.01
Amounts attributable to Alliant Energy common shareowners:
Income from continuing operations, net of tax	302.3	289.3	110.1
Income (loss) from discontinued operations, net of tax	1.3	(1.7)	0.9
Net income attributable to common shareowners	303.6	287.6	111.0
Dividends declared per common share	$ 1.70	$ 1.58	$ 1.50
IPL [Member]
Utility:
Electric	1,408.3	1,464.3	1,315.6
Gas	276.3	274.3	308.8
Other	55.5	57.2	83.6
Total operating revenues	1,740.1	1,795.8	1,708.0
Utility:
Electric production fuel and energy purchases	383.1	418.0	440.1
Purchased electric capacity	147.7	145.0	136.5
Electric transmission service	219.2	179.1	131.2
Cost of gas sold	175.6	178.7	209.8
Other operation and maintenance	375.0	384.5	365.4
Depreciation and amortization	179.1	177.0	152.8
Taxes other than income taxes	52.0	51.6	50.5
Total operating expenses	1,531.7	1,533.9	1,486.3
Operating income	208.4	261.9	221.7
Interest expense and other:
Interest expense	78.7	82.2	76.5
Allowance for funds used during construction	(5.8)	(5.5)	(34.0)
Interest income and other	(0.2)	(0.5)	(0.8)
Total interest expense and other	72.7	76.2	41.7
Income from continuing operations before income taxes	135.7	185.7	180.0
Income tax expense (benefit)	(3.6)	42.3	27.0
Net income	139.3	143.4	153.0
Preferred dividend requirements of subsidiaries	15.0	15.4	15.4
Net income attributable to common shareowners	124.3	128.0	137.6
Amounts attributable to Alliant Energy common shareowners:
Net income attributable to common shareowners	124.3	128.0	137.6
WPL [Member]
Utility:
Electric	1,227.5	1,209.9	1,160.3
Gas	200.4	206.3	216.5
Other	6.5	7.4	9.3
Total operating revenues	1,434.4	1,423.6	1,386.1
Utility:
Electric production fuel and energy purchases	381.4	401.2	451.3
Purchased electric capacity	109.5	134.7	144.6
Electric transmission service	104.6	100.4	94.2
Cost of gas sold	119.6	125.3	138.1
Other operation and maintenance	255.2	232.7	234.3
Depreciation and amortization	140.1	108.6	115.4
Taxes other than income taxes	43.6	41.9	41.2
Total operating expenses	1,154.0	1,144.8	1,219.1
Operating income	280.4	278.8	167.0
Interest expense and other:
Interest expense	79.9	78.6	74.8
Equity (income)/loss from unconsolidated investments, net	(38.7)	(37.8)	(37.0)
Allowance for funds used during construction	(6.2)	(12.5)	(5.7)
Interest income and other		(0.1)	(0.4)
Total interest expense and other	35.0	28.2	31.7
Income from continuing operations before income taxes	245.4	250.6	135.3
Income tax expense (benefit)	81.9	98.3	45.8
Net income	163.5	152.3	89.5
Preferred dividend requirements of subsidiaries	3.3	3.3	3.3
Net income attributable to common shareowners	160.2	149.0	86.2
Amounts attributable to Alliant Energy common shareowners:
Net income attributable to common shareowners	$ 160.2	$ 149.0	$ 86.2