Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Utility:
Electric plant in service	$ 9,106.2	$ 9,070.7
Gas plant in service	884.8	878.4
Other plant in service	509.7	506.2
Accumulated depreciation	(3,573.4)	(3,513.0)
Net plant	6,927.3	6,942.3
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	171.2	130.4
Ottumwa Generating Station Unit 1 emission controls (IPL)	93.7	73.7
George Neal Generating Station Units 3 and 4 emission controls (IPL)	82.4	66.9
Other	166.0	147.8
Other, less accumulated depreciation	21.1	21.2
Total utility	7,461.7	7,382.3
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	255.8	258.6
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	196.1	197.1
Total non-regulated and other	451.9	455.7
Total property, plant and equipment	7,913.6	7,838.0
Current assets:
Cash and cash equivalents	53.4	21.2
Accounts receivable, less allowance for doubtful accounts:
Customer	98.9	94.9
Unbilled utility revenues	73.8	81.4
Other	220.6	209.4
Production fuel, at weighted average cost	99.8	103.1
Materials and supplies, at weighted average cost	64.9	63.1
Gas stored underground, at weighted average cost	9.0	37.7
Deferred income tax assets	92.5	170.2
Other	157.5	213.3
Total current assets	870.4	994.3
Investments:
Investment in American Transmission Company LLC	259.8	257.0
Other	61.8	62.0
Total investments	321.6	319.0
Other assets:
Regulatory assets	1,519.4	1,528.9
Deferred charges and other	108.9	105.3
Total other assets	1,628.3	1,634.2
Total assets	10,733.9	10,785.5
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	1.1	1.1
Additional paid-in capital	1,504.4	1,511.2
Retained earnings	1,648.4	1,630.7
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust - 212,456 and 216,030 shares at a weighted average cost of $34.07 and $33.61 per share	(7.2)	(7.3)
Total common equity	3,145.9	3,134.9
Cumulative preferred stock (Interstate Power and Light Company)	200.0	145.1
Noncontrolling interest	1.8	1.8
Total equity	3,347.7	3,281.8
Cumulative preferred stock (Wisconsin Power and Light Company)	0	60.0
Long-term debt, net (excluding current portion)	3,121.9	3,136.6
Total capitalization	6,469.6	6,478.4
Current liabilities:
Current maturities of long-term debt	1.5	1.5
Commercial paper	243.4	217.5
Accounts payable	368.6	339.3
Regulatory liabilities	228.5	189.7
Other	196.1	272.0
Total current liabilities	1,038.1	1,020.0
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,901.4	1,934.2
Regulatory liabilities	707.2	726.4
Pension and other benefit obligations	360.3	364.0
Other	257.3	262.5
Total long-term liabilities and deferred credits	3,226.2	3,287.1
Commitments and contingencies (Note 12)
Total capitalization and liabilities	10,733.9	10,785.5
IPL [Member]
Utility:
Electric plant in service	4,836.0	4,815.2
Gas plant in service	444.3	441.4
Other plant in service	292.1	289.1
Accumulated depreciation	(1,959.5)	(1,930.7)
Net plant	3,612.9	3,615.0
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	93.7	73.7
George Neal Generating Station Units 3 and 4 emission controls (IPL)	82.4	66.9
Other	94.7	82.8
Other, less accumulated depreciation	19.7	19.8
Non-regulated and other:
Total property, plant and equipment	3,903.4	3,858.2
Current assets:
Cash and cash equivalents	4.7	4.5
Accounts receivable, less allowance for doubtful accounts:
Customer	165.1	95.0
Income tax refunds receivable	23.6	14.9
Production fuel, at weighted average cost	74.4	75.2
Materials and supplies, at weighted average cost	34.6	33.3
Gas stored underground, at weighted average cost	2.2	17.2
Deferred income tax assets	58.7	79.3
Other	54.3	72.2
Total current assets	417.6	391.6
Investments:
Total investments	17.6	17.6
Other assets:
Regulatory assets	1,171.5	1,170.3
Deferred charges and other	20.8	19.3
Total other assets	1,192.3	1,189.6
Total assets	5,530.9	5,457.0
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	33.4	33.4
Additional paid-in capital	1,062.8	1,037.8
Retained earnings	439.3	448.0
Total common equity	1,535.5	1,519.2
Cumulative preferred stock (Interstate Power and Light Company)	200.0	145.1
Total equity	1,735.5	1,664.3
Long-term debt, net (excluding current portion)	1,344.5	1,359.5
Total capitalization	3,080.0	3,023.8
Current liabilities:
Commercial paper	0	26.3
Accounts payable	204.2	163.2
Accounts payable to associated companies	37.4	29.3
Regulatory liabilities	164.9	130.1
Other	114.2	119.9
Total current liabilities	520.7	468.8
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,079.6	1,087.3
Regulatory liabilities	550.3	571.3
Pension and other benefit obligations	122.1	122.9
Other	178.2	182.9
Total long-term liabilities and deferred credits	1,930.2	1,964.4
Commitments and contingencies (Note 12)
Total capitalization and liabilities	5,530.9	5,457.0
WPL [Member]
Utility:
Electric plant in service	4,270.2	4,255.5
Gas plant in service	440.5	437.0
Other plant in service	217.6	217.1
Accumulated depreciation	(1,613.9)	(1,582.3)
Net plant	3,314.4	3,327.3
Leased Sheboygan Falls Energy Facility, less accumulated amortization	75.5	77.0
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	171.2	130.4
Other	71.3	65.0
Other, less accumulated depreciation	1.4	1.4
Non-regulated and other:
Total property, plant and equipment	3,633.8	3,601.1
Current assets:
Cash and cash equivalents	8.4	0.7
Accounts receivable, less allowance for doubtful accounts:
Customer	89.6	83.3
Unbilled utility revenues	73.8	81.4
Other	52.1	48.5
Production fuel, at weighted average cost	25.4	27.9
Materials and supplies, at weighted average cost	29.0	28.5
Gas stored underground, at weighted average cost	6.8	20.5
Regulatory assets	26.7	35.9
Deferred income tax assets	22.6	85.6
Other	63.7	56.4
Total current assets	398.1	468.7
Investments:
Investment in American Transmission Company LLC	259.8	257.0
Other	19.2	19.6
Total investments	279.0	276.6
Other assets:
Regulatory assets	347.9	358.6
Deferred charges and other	58.7	57.6
Total other assets	406.6	416.2
Total assets	4,717.5	4,762.6
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	66.2	66.2
Additional paid-in capital	959.1	959.2
Retained earnings	570.5	557.6
Total common equity	1,595.8	1,583.0
Cumulative preferred stock (Wisconsin Power and Light Company)	0	60.0
Long-term debt, net (excluding current portion)	1,331.7	1,331.5
Total capitalization	2,927.5	2,974.5
Current liabilities:
Commercial paper	160.0	86.6
Accounts payable	115.8	126.4
Accounts payable to associated companies	16.3	13.2
Accrued taxes	4.0	28.3
Other	126.7	131.0
Total current liabilities	422.8	385.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	811.2	844.1
Regulatory liabilities	156.9	155.1
Capital lease obligations - Sheboygan Falls Energy Facility	98.0	99.1
Pension and other benefit obligations	159.5	159.7
Other	141.6	144.6
Total long-term liabilities and deferred credits	1,367.2	1,402.6
Commitments and contingencies (Note 12)
Total capitalization and liabilities	$ 4,717.5	$ 4,762.6