Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Utility:
Electric plant in service	$ 9,165.1	$ 9,070.7
Gas plant in service	892.4	878.4
Other plant in service	516.0	506.2
Accumulated depreciation	(3,632.0)	(3,513.0)
Net plant	6,941.5	6,942.3
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	215.3	130.4
Ottumwa Generating Station Unit 1 emission controls (IPL)	110.8	73.7
George Neal Generating Station Units 3 and 4 emission controls (IPL)	95.0	66.9
Other	189.3	147.8
Other, less accumulated depreciation	21.0	21.2
Total utility	7,572.9	7,382.3
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	253.5	258.6
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	200.8	197.1
Total non-regulated and other	454.3	455.7
Total property, plant and equipment	8,027.2	7,838.0
Current assets:
Cash and cash equivalents	11.5	21.2
Accounts receivable, less allowance for doubtful accounts:
Customer	85.0	94.9
Unbilled utility revenues	69.3	81.4
Other	190.1	209.4
Production fuel, at weighted average cost	99.7	103.1
Materials and supplies, at weighted average cost	68.3	63.1
Gas stored underground, at weighted average cost	23.2	37.7
Regulatory assets	78.3	83.5
Deferred income tax assets	142.3	170.2
Other	125.7	129.8
Total current assets	893.4	994.3
Investments:
Investment in American Transmission Company LLC	264.4	257.0
Other	56.9	62.0
Total investments	321.3	319.0
Other assets:
Regulatory assets	1,533.8	1,528.9
Deferred charges and other	111.9	105.3
Total other assets	1,645.7	1,634.2
Total assets	10,887.6	10,785.5
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	1.1	1.1
Additional paid-in capital	1,506.0	1,511.2
Retained earnings	1,661.7	1,630.7
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust - 217,404 and 216,030 shares at a weighted average cost of $34.48 and $33.61 per share	(7.5)	(7.3)
Total common equity	3,160.5	3,134.9
Cumulative preferred stock (Interstate Power and Light Company)	200.0	145.1
Noncontrolling interest	1.8	1.8
Total equity	3,362.3	3,281.8
Cumulative preferred stock (Wisconsin Power and Light Company)	0	60.0
Long-term debt, net (excluding current portion)	3,141.4	3,136.6
Total capitalization	6,503.7	6,478.4
Current liabilities:
Current maturities of long-term debt	1.5	1.5
Commercial paper	223.1	217.5
Accounts payable	391.1	339.3
Regulatory liabilities	212.8	189.7
Other	244.0	272.0
Total current liabilities	1,072.5	1,020.0
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,991.1	1,934.2
Regulatory liabilities	685.3	726.4
Pension and other benefit obligations	359.2	364.0
Other	275.8	262.5
Total long-term liabilities and deferred credits	3,311.4	3,287.1
Commitments and contingencies (Note 12)
Total capitalization and liabilities	10,887.6	10,785.5
IPL [Member]
Utility:
Electric plant in service	4,862.4	4,815.2
Gas plant in service	447.3	441.4
Other plant in service	294.2	289.1
Accumulated depreciation	(1,983.3)	(1,930.7)
Net plant	3,620.6	3,615.0
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	110.8	73.7
George Neal Generating Station Units 3 and 4 emission controls (IPL)	95.0	66.9
Other	116.7	82.8
Other, less accumulated depreciation	19.8	19.8
Non-regulated and other:
Total property, plant and equipment	3,962.9	3,858.2
Current assets:
Cash and cash equivalents	5.1	4.5
Accounts receivable, less allowance for doubtful accounts:
Customer	115.7	95.0
Income tax refunds receivable	8.5	14.9
Production fuel, at weighted average cost	71.1	75.2
Materials and supplies, at weighted average cost	37.7	33.3
Gas stored underground, at weighted average cost	10.2	17.2
Regulatory assets	46.0	47.6
Deferred income tax assets	86.2	79.3
Other	50.5	24.6
Total current assets	431.0	391.6
Investments:
Total investments	18.0	17.6
Other assets:
Regulatory assets	1,178.2	1,170.3
Deferred charges and other	21.0	19.3
Total other assets	1,199.2	1,189.6
Total assets	5,611.1	5,457.0
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	33.4	33.4
Additional paid-in capital	1,092.9	1,037.8
Retained earnings	429.6	448.0
Total common equity	1,555.9	1,519.2
Cumulative preferred stock (Interstate Power and Light Company)	200.0	145.1
Total equity	1,755.9	1,664.3
Long-term debt, net (excluding current portion)	1,364.7	1,359.5
Total capitalization	3,120.6	3,023.8
Current liabilities:
Commercial paper	0	26.3
Accounts payable	213.5	163.2
Accounts payable to associated companies	30.4	29.3
Regulatory liabilities	146.3	130.1
Other	137.6	119.9
Total current liabilities	527.8	468.8
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,131.6	1,087.3
Regulatory liabilities	526.1	571.3
Pension and other benefit obligations	121.1	122.9
Other	183.9	182.9
Total long-term liabilities and deferred credits	1,962.7	1,964.4
Commitments and contingencies (Note 12)
Total capitalization and liabilities	5,611.1	5,457.0
WPL [Member]
Utility:
Electric plant in service	4,302.7	4,255.5
Gas plant in service	445.1	437.0
Other plant in service	221.8	217.1
Accumulated depreciation	(1,648.7)	(1,582.3)
Net plant	3,320.9	3,327.3
Leased Sheboygan Falls Energy Facility, less accumulated amortization	74.0	77.0
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	215.3	130.4
Other	72.6	65.0
Other, less accumulated depreciation	1.2	1.4
Non-regulated and other:
Total property, plant and equipment	3,684.0	3,601.1
Current assets:
Cash and cash equivalents	2.5	0.7
Accounts receivable, less allowance for doubtful accounts:
Customer	74.9	83.3
Unbilled utility revenues	69.3	81.4
Other	45.8	48.5
Production fuel, at weighted average cost	28.6	27.9
Materials and supplies, at weighted average cost	29.0	28.5
Gas stored underground, at weighted average cost	13.0	20.5
Regulatory assets	32.3	35.9
Deferred income tax assets	43.0	85.6
Other	57.3	56.4
Total current assets	395.7	468.7
Investments:
Investment in American Transmission Company LLC	264.4	257.0
Other	19.6	19.6
Total investments	284.0	276.6
Other assets:
Regulatory assets	355.6	358.6
Deferred charges and other	63.0	57.6
Total other assets	418.6	416.2
Total assets	4,782.3	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	575.8	557.6
Total common equity	1,601.1	1,583.0
Cumulative preferred stock (Wisconsin Power and Light Company)	0	60.0
Long-term debt, net (excluding current portion)	1,331.9	1,331.5
Total capitalization	2,933.0	2,974.5
Current liabilities:
Commercial paper	148.9	86.6
Accounts payable	124.0	126.4
Accounts payable to associated companies	13.2	13.2
Regulatory liabilities	66.5	59.6
Accrued taxes	11.0	28.3
Other	72.2	71.4
Total current liabilities	435.8	385.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	843.8	844.1
Regulatory liabilities	159.2	155.1
Capital lease obligations - Sheboygan Falls Energy Facility	96.9	99.1
Pension and other benefit obligations	158.0	159.7
Other	155.6	144.6
Total long-term liabilities and deferred credits	1,413.5	1,402.6
Commitments and contingencies (Note 12)
Total capitalization and liabilities	$ 4,782.3	$ 4,762.6