Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Utility:
Electric plant in service	$ 8,335.4	$ 8,165.4
Gas plant in service	869.4	852.9
Other plant in service	512.5	510.1
Accumulated depreciation	(3,351.5)	(3,206.0)
Net plant	6,365.8	6,322.4
Construction work in progress:
Edgewater Generating Station Unit 5 emission controls (Wisconsin Power and Light Company)	124.0	77.7
Columbia Energy Center Units 1 and 2 emission controls (Wisconsin Power and Light Company)	91.6	9.0
George Neal Generating Station Units 3 and 4 emission controls (Interstate Power and Light Company)	53.5	8.3
Ottumwa Generating Station Unit 1 emission controls (Interstate Power and Light Company)	53.3	7.7
Other	159.1	154.5
Other, less accumulated depreciation	21.3	34.9
Total utility	6,868.6	6,614.5
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	326.5	270.6
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	189.5	148.2
Total non-regulated and other	516.0	418.8
Total property, plant and equipment	7,384.6	7,033.3
Current assets:
Cash and cash equivalents	41.1	11.4
Accounts receivable:
Customer, less allowance for doubtful accounts	93.9	88.1
Unbilled utility revenues	59.8	75.1
Other, less allowance for doubtful accounts	239.7	114.9
Income tax refunds receivable	43.9	39.1
Production fuel, at weighted average cost	110.5	101.9
Materials and supplies, at weighted average cost	61.4	58.5
Gas stored underground, at weighted average cost	37.3	57.7
Regulatory assets	92.9	103.6
Derivative assets	35.9	12.7
Prepaid gross receipts tax	29.1	40.2
Deferred income tax assets	90.0	22.8
Assets held for sale	51.4	119.6
Prepayments and other	41.7	25.0
Total current assets	1,028.6	870.6
Investments:
Investment in American Transmission Company LLC	253.2	238.8
Other	60.9	61.9
Total investments	314.1	300.7
Other assets:
Regulatory assets	1,425.5	1,391.4
Deferred charges and other	98.1	91.9
Total other assets	1,523.6	1,483.3
Total assets	10,250.9	9,687.9
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	1.1	1.1
Additional paid-in capital	1,512.1	1,510.8
Retained earnings	1,612.1	1,510.2
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust - 261,057 and 262,735 shares at a weighted average cost of $32.68 and $31.68 per share	(8.5)	(8.3)
Total common equity	3,116.0	3,013.0
Cumulative preferred stock (Interstate Power and Light Company)	145.1	145.1
Noncontrolling interest	1.7	1.8
Total equity	3,262.8	3,159.9
Cumulative preferred stock (Wisconsin Power and Light Company)	60.0	60.0
Long-term debt, net (excluding current portion)	2,828.1	2,703.1
Total capitalization	6,150.9	5,923.0
Current liabilities:
Current maturities of long-term debt	1.4	1.4
Commercial paper	70.4	102.8
Accounts payable	418.0	267.8
Regulatory liabilities	168.5	164.7
Accrued taxes	37.4	46.9
Accrued interest	46.6	46.6
Derivative liabilities	37.8	55.9
Liabilities held for sale	66.1	62.1
Other	99.3	107.0
Total current liabilities	945.5	855.2
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,836.7	1,592.2
Regulatory liabilities	737.9	745.4
Pension and other benefit obligations	310.8	312.7
Other	269.1	259.4
Total long-term liabilities and deferred credits	3,154.5	2,909.7
Total capitalization and liabilities	10,250.9	9,687.9
IPL [Member]
Utility:
Electric plant in service	4,767.4	4,684.0
Gas plant in service	436.9	428.2
Steam plant in service	34.9	34.9
Other plant in service	253.6	246.4
Accumulated depreciation	(1,907.0)	(1,833.8)
Net plant	3,585.8	3,559.7
Construction work in progress:
George Neal Generating Station Units 3 and 4 emission controls (Interstate Power and Light Company)	53.5	8.3
Ottumwa Generating Station Unit 1 emission controls (Interstate Power and Light Company)	53.3	7.7
Other	87.4	80.6
Other, less accumulated depreciation	19.8	19.8
Non-regulated and other:
Total property, plant and equipment	3,799.8	3,676.1
Current assets:
Cash and cash equivalents	5.3	2.1
Accounts receivable:
Customer, less allowance for doubtful accounts	183.4	75.2
Income tax refunds receivable	20.7	28.4
Production fuel, at weighted average cost	76.1	67.7
Materials and supplies, at weighted average cost	33.3	31.5
Gas stored underground, at weighted average cost	19.6	25.5
Regulatory assets	55.2	59.0
Derivative assets	25.9	9.2
Deferred income tax assets	35.9	13.5
Prepayments and other	11.9	11.0
Total current assets	467.3	323.1
Investments:
Total investments	17.0	16.8
Other assets:
Regulatory assets	1,095.8	1,058.3
Deferred charges and other	19.3	19.2
Total other assets	1,115.1	1,077.5
Total assets	5,399.2	5,093.5
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	33.4	33.4
Additional paid-in capital	1,007.8	927.7
Retained earnings	456.9	433.3
Total common equity	1,498.1	1,394.4
Cumulative preferred stock (Interstate Power and Light Company)	145.1	145.1
Total equity	1,643.2	1,539.5
Long-term debt, net (excluding current portion)	1,359.3	1,309.0
Total capitalization	3,002.5	2,848.5
Current liabilities:
Commercial paper	0	7.1
Accounts payable	231.4	118.2
Accounts payable to associated companies	35.1	36.7
Regulatory liabilities	111.0	137.1
Accrued taxes	38.0	43.8
Accrued interest	22.9	22.8
Derivative liabilities	17.9	24.5
Other	34.2	32.3
Total current liabilities	490.5	422.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,039.4	936.9
Regulatory liabilities	577.1	584.2
Pension and other benefit obligations	99.8	101.9
Other	189.9	199.5
Total long-term liabilities and deferred credits	1,906.2	1,822.5
Total capitalization and liabilities	5,399.2	5,093.5
WPL [Member]
Utility:
Electric plant in service	3,568.0	3,481.4
Gas plant in service	432.5	424.7
Other plant in service	224.0	228.8
Accumulated depreciation	(1,444.5)	(1,372.2)
Net plant	2,780.0	2,762.7
Leased Sheboygan Falls Energy Facility, less accumulated amortization	78.6	83.2
Construction work in progress:
Edgewater Generating Station Unit 5 emission controls (Wisconsin Power and Light Company)	124.0	77.7
Columbia Energy Center Units 1 and 2 emission controls (Wisconsin Power and Light Company)	91.6	9.0
Other	71.7	73.9
Other, less accumulated depreciation	1.5	15.1
Non-regulated and other:
Total property, plant and equipment	3,147.4	3,021.6
Current assets:
Cash and cash equivalents	16.2	2.7
Accounts receivable:
Customer, less allowance for doubtful accounts	84.6	76.2
Unbilled utility revenues	59.8	75.1
Other, less allowance for doubtful accounts	44.4	38.2
Income tax refunds receivable	23.0	0.7
Production fuel, at weighted average cost	34.4	34.2
Materials and supplies, at weighted average cost	26.7	25.7
Gas stored underground, at weighted average cost	17.7	32.2
Regulatory assets	37.7	44.6
Derivative assets	10.0	3.5
Prepaid gross receipts tax	29.1	40.2
Deferred income tax assets	50.3	6.0
Prepayments and other	20.8	6.7
Total current assets	454.7	386.0
Investments:
Investment in American Transmission Company LLC	253.2	238.8
Other	19.3	19.8
Total investments	272.5	258.6
Other assets:
Regulatory assets	329.7	333.1
Deferred charges and other	53.6	44.7
Total other assets	383.3	377.8
Total assets	4,257.9	4,044.0
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	66.2	66.2
Additional paid-in capital	869.2	869.0
Retained earnings	545.3	507.2
Total common equity	1,480.7	1,442.4
Cumulative preferred stock (Wisconsin Power and Light Company)	60.0	60.0
Long-term debt, net (excluding current portion)	1,082.5	1,082.2
Total capitalization	2,623.2	2,584.6
Current liabilities:
Commercial paper	0	25.7
Accounts payable	133.5	98.5
Accounts payable to associated companies	20.5	20.5
Regulatory liabilities	57.5	27.6
Accrued interest	18.1	21.6
Derivative liabilities	19.9	31.4
Other	38.3	32.3
Total current liabilities	287.8	257.6
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	809.3	672.5
Regulatory liabilities	160.8	161.2
Capital lease obligations - Sheboygan Falls Energy Facility	100.2	103.3
Pension and other benefit obligations	127.8	128.0
Other	148.8	136.8
Total long-term liabilities and deferred credits	1,346.9	1,201.8
Total capitalization and liabilities	$ 4,257.9	$ 4,044.0