Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Utility:
Electric plant in service	$ 9,070.7	$ 8,165.4
Gas plant in service	878.4	852.9
Other plant in service	506.2	510.1
Accumulated depreciation (accum. depr.)	(3,513.0)	(3,206.0)
Net plant	6,942.3	6,322.4
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	130.4	9.0
Ottumwa Generating Station Unit 1 emission controls (IPL)	73.7	7.7
George Neal Generating Station Units 3 and 4 emission controls (IPL)	66.9	8.3
Other	147.8	232.2
Other, less accum. depr.	21.2	34.9
Total utility	7,382.3	6,614.5
Non-regulated and other:
Non-regulated Generation, less accum. depr.	258.6	270.6
Alliant Energy Corporate Services, Inc. and other, less accum. depr.	197.1	148.2
Total non-regulated and other	455.7	418.8
Total property, plant and equipment	7,838.0	7,033.3
Current assets:
Cash and cash equivalents	21.2	11.4
Accounts receivable, less allowance for doubtul accounts:
Customer	94.9	88.1
Unbilled utility revenues	81.4	75.1
Other	209.4	114.9
Income tax refunds receivable	18.2	39.1
Production fuel, at weighted average cost	103.1	101.9
Materials and supplies, at weighted average cost	63.1	58.5
Gas stored underground, at weighted average cost	37.7	57.7
Regulatory assets	83.5	103.6
Prepaid gross receipts tax	40.4	40.2
Deferred income tax assets	170.2	22.8
Assets held for sale	27.9	119.6
Prepayments and other	43.3	37.7
Total current assets	994.3	870.6
Investments:
Investment in American Transmission Company LLC	257.0	238.8
Other	62.0	61.9
Total investments	319.0	300.7
Other assets:
Regulatory assets	1,528.9	1,391.4
Deferred charges and other	105.3	91.9
Total other assets	1,634.2	1,483.3
Total assets	10,785.5	9,687.9
Capitalization:
Common stock	1.1	1.1
Additional paid-in capital	1,511.2	1,510.8
Retained earnings	1,630.7	1,510.2
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust - 216,030 and 262,735 shares at a weighted average cost of $33.61 and $31.68 per share	(7.3)	(8.3)
Total common equity	3,134.9	3,013.0
Cumulative preferred stock of Interstate Power and Light Company	145.1	145.1
Noncontrolling interest	1.8	1.8
Total equity	3,281.8	3,159.9
Cumulative preferred stock of Wisconsin Power and Light Company	60.0	60.0
Long-term debt, net (excluding current portion)	3,136.6	2,703.1
Total capitalization	6,478.4	5,923.0
Current liabilities:
Current maturities of long-term debt	1.5	1.4
Commercial paper	217.5	102.8
Accounts payable	339.3	267.8
Regulatory liabilities	189.7	164.7
Accrued taxes	48.0	46.9
Accrued interest	48.0	46.6
Derivative liabilities	31.1	55.9
Liabilities held for sale	31.4	62.1
Other	113.5	107.0
Total current liabilities	1,020.0	855.2
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,934.2	1,592.2
Regulatory liabilities	726.4	745.4
Pension and other benefit obligations	364.0	312.7
Other	262.5	259.4
Total long-term liabilities and deferred credits	3,287.1	2,909.7
Total capitalization and liabilities	10,785.5	9,687.9
IPL [Member]
Utility:
Electric plant in service	4,815.2	4,684.0
Gas plant in service	441.4	428.2
Other plant in service	289.1	281.3
Accumulated depreciation (accum. depr.)	(1,930.7)	(1,833.8)
Net plant	3,615.0	3,559.7
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	73.7	7.7
George Neal Generating Station Units 3 and 4 emission controls (IPL)	66.9	8.3
Other	82.8	80.6
Other, less accum. depr.	19.8	19.8
Non-regulated and other:
Total property, plant and equipment	3,858.2	3,676.1
Current assets:
Cash and cash equivalents	4.5	2.1
Accounts receivable, less allowance for doubtul accounts:
Customer	95.0	75.2
Income tax refunds receivable	14.9	28.4
Production fuel, at weighted average cost	75.2	67.7
Materials and supplies, at weighted average cost	33.3	31.5
Gas stored underground, at weighted average cost	17.2	25.5
Regulatory assets	47.6	59.0
Deferred income tax assets	79.3	13.5
Prepayments and other	24.6	20.2
Total current assets	391.6	323.1
Investments:
Total investments	17.6	16.8
Other assets:
Regulatory assets	1,170.3	1,058.3
Deferred charges and other	19.3	19.2
Total other assets	1,189.6	1,077.5
Total assets	5,457.0	5,093.5
Capitalization:
Common stock	33.4	33.4
Additional paid-in capital	1,037.8	927.7
Retained earnings	448.0	433.3
Total common equity	1,519.2	1,394.4
Cumulative preferred stock of Interstate Power and Light Company	145.1	145.1
Total equity	1,664.3	1,539.5
Long-term debt, net (excluding current portion)	1,359.5	1,309.0
Total capitalization	3,023.8	2,848.5
Current liabilities:
Current maturities of long-term debt	0	0
Commercial paper	26.3	7.1
Accounts payable	163.2	118.2
Accounts payable to associated companies	29.3	36.7
Regulatory liabilities	130.1	137.1
Accrued taxes	46.8	43.8
Accrued interest	22.8	22.8
Derivative liabilities	14.1	24.5
Other	36.2	32.3
Total current liabilities	468.8	422.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,087.3	936.9
Regulatory liabilities	571.3	584.2
Pension and other benefit obligations	122.9	101.9
Other	182.9	199.5
Total long-term liabilities and deferred credits	1,964.4	1,822.5
Total capitalization and liabilities	5,457.0	5,093.5
WPL [Member]
Utility:
Electric plant in service	4,255.5	3,481.4
Gas plant in service	437.0	424.7
Other plant in service	217.1	228.8
Accumulated depreciation (accum. depr.)	(1,582.3)	(1,372.2)
Net plant	3,327.3	2,762.7
Leased Sheboygan Falls Energy Facility, less accumulated amortization of $46.7 and $40.6	77.0	83.2
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	130.4	9.0
Other	65.0	151.6
Other, less accum. depr.	1.4	15.1
Non-regulated and other:
Total property, plant and equipment	3,601.1	3,021.6
Current assets:
Cash and cash equivalents	0.7	2.7
Accounts receivable, less allowance for doubtul accounts:
Customer	83.3	76.2
Unbilled utility revenues	81.4	75.1
Other	48.5	38.2
Production fuel, at weighted average cost	27.9	34.2
Materials and supplies, at weighted average cost	28.5	25.7
Gas stored underground, at weighted average cost	20.5	32.2
Regulatory assets	35.9	44.6
Prepaid gross receipts tax	40.4	40.2
Deferred income tax assets	85.6	6.0
Prepayments and other	16.0	10.9
Total current assets	468.7	386.0
Investments:
Investment in American Transmission Company LLC	257.0	238.8
Other	19.6	19.8
Total investments	276.6	258.6
Other assets:
Regulatory assets	358.6	333.1
Deferred charges and other	57.6	44.7
Total other assets	416.2	377.8
Total assets	4,762.6	4,044.0
Capitalization:
Common stock	66.2	66.2
Additional paid-in capital	959.2	869.0
Retained earnings	557.6	507.2
Total common equity	1,583.0	1,442.4
Cumulative preferred stock of Wisconsin Power and Light Company	60.0	60.0
Long-term debt, net (excluding current portion)	1,331.5	1,082.2
Total capitalization	2,974.5	2,584.6
Current liabilities:
Current maturities of long-term debt	0	0
Commercial paper	86.6	25.7
Accounts payable	126.4	98.5
Accounts payable to associated companies	13.2	20.5
Regulatory liabilities	59.6	27.6
Accrued taxes	28.3	9.1
Accrued interest	22.2	21.6
Derivative liabilities	17.0	31.4
Other	32.2	23.2
Total current liabilities	385.5	257.6
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	844.1	672.5
Regulatory liabilities	155.1	161.2
Capital lease obligations - Sheboygan Falls Energy Facility	99.1	103.3
Pension and other benefit obligations	159.7	128.0
Other	144.6	136.8
Total long-term liabilities and deferred credits	1,402.6	1,201.8
Total capitalization and liabilities	$ 4,762.6	$ 4,044.0