Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Utility:
Electric plant in service	$ 8,278.1	$ 8,165.4
Gas plant in service	860.1	852.9
Other plant in service	514.2	510.1
Accumulated depreciation	(3,298.9)	(3,206.0)
Net plant	6,353.5	6,322.4
Construction work in progress:
Edgewater Generating Station Unit 5 emission controls (Wisconsin Power and Light Company)	104.0	77.7
Columbia Energy Center Units 1 and 2 emission controls (Wisconsin Power and Light Company)	54.4	9.0
Other	219.9	170.5
Other, less accumulated depreciation	21.5	34.9
Total utility	6,753.3	6,614.5
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	302.4	270.6
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	192.3	148.2
Total non-regulated and other	494.7	418.8
Total property, plant and equipment	7,248.0	7,033.3
Current assets:
Cash and cash equivalents	30.8	11.4
Accounts receivable:
Customer, less allowance for doubtful accounts	81.7	88.1
Unbilled utility revenues	76.8	75.1
Other, less allowance for doubtful accounts	147.3	114.9
Income tax refunds receivable	29.7	39.1
Production fuel, at weighted average cost	116.7	101.9
Materials and supplies, at weighted average cost	61.1	58.5
Gas stored underground, at weighted average cost	28.0	57.7
Regulatory assets	98.9	103.6
Derivative assets	40.7	12.7
Prepaid gross receipts tax	37.3	40.2
Deferred income tax assets	82.9	22.8
Assets held for sale	53.6	119.6
Prepayments and other	34.9	25.0
Total current assets	920.4	870.6
Investments:
Investment in American Transmission Company LLC	247.0	238.8
Other	61.4	61.9
Total investments	308.4	300.7
Other assets:
Regulatory assets	1,380.6	1,391.4
Deferred charges and other	76.8	91.9
Total other assets	1,457.4	1,483.3
Total assets	9,934.2	9,687.9
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	1.1	1.1
Additional paid-in capital	1,511.1	1,510.8
Retained earnings	1,511.3	1,510.2
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust - 255,319 and 262,735 shares at a weighted average cost of $32.37 and $31.68 per share	(8.3)	(8.3)
Total common equity	3,014.4	3,013.0
Cumulative preferred stock (Interstate Power and Light Company)	145.1	145.1
Noncontrolling interest	1.8	1.8
Total equity	3,161.3	3,159.9
Cumulative preferred stock (Wisconsin Power and Light Company)	60.0	60.0
Long-term debt, net (excluding current portion)	2,752.8	2,703.1
Total capitalization	5,974.1	5,923.0
Current liabilities:
Current maturities of long-term debt	1.4	1.4
Commercial paper	162.8	102.8
Accounts payable	337.6	267.8
Regulatory liabilities	149.9	164.7
Accrued taxes	46.3	46.9
Accrued interest	46.6	46.6
Derivative liabilities	48.4	55.9
Liabilities held for sale	70.9	62.1
Other	93.4	107.0
Total current liabilities	957.3	855.2
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,736.1	1,592.2
Regulatory liabilities	711.7	745.4
Pension and other benefit obligations	309.9	312.7
Other	245.1	259.4
Total long-term liabilities and deferred credits	3,002.8	2,909.7
Total capitalization and liabilities	9,934.2	9,687.9
IPL [Member]
Utility:
Electric plant in service	4,736.5	4,684.0
Gas plant in service	431.1	428.2
Steam plant in service	34.9	34.9
Other plant in service	251.8	246.4
Accumulated depreciation	(1,876.2)	(1,833.8)
Net plant	3,578.1	3,559.7
Construction work in progress:
Other	144.5	96.6
Other, less accumulated depreciation	19.7	19.8
Non-regulated and other:
Total property, plant and equipment	3,742.3	3,676.1
Current assets:
Cash and cash equivalents	2.8	2.1
Accounts receivable:
Customer, less allowance for doubtful accounts	102.4	75.2
Income tax refunds receivable	16.3	28.4
Production fuel, at weighted average cost	75.9	67.7
Materials and supplies, at weighted average cost	32.8	31.5
Gas stored underground, at weighted average cost	7.9	25.5
Regulatory assets	60.1	59.0
Derivative assets	29.8	9.2
Deferred income tax assets	34.9	13.5
Prepayments and other	11.3	11.0
Total current assets	374.2	323.1
Investments:
Total investments	17.0	16.8
Other assets:
Regulatory assets	1,050.3	1,058.3
Deferred charges and other	17.7	19.2
Total other assets	1,068.0	1,077.5
Total assets	5,201.5	5,093.5
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	33.4	33.4
Additional paid-in capital	977.8	927.7
Retained earnings	384.4	433.3
Total common equity	1,395.6	1,394.4
Cumulative preferred stock (Interstate Power and Light Company)	145.1	145.1
Total equity	1,540.7	1,539.5
Long-term debt, net (excluding current portion)	1,359.3	1,309.0
Total capitalization	2,900.0	2,848.5
Current liabilities:
Commercial paper	15.4	7.1
Accounts payable	171.8	118.2
Accounts payable to associated companies	33.7	36.7
Regulatory liabilities	100.9	137.1
Accrued taxes	67.2	43.8
Accrued interest	22.8	22.8
Derivative liabilities	23.2	24.5
Other	31.8	32.3
Total current liabilities	466.8	422.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	992.6	936.9
Regulatory liabilities	555.0	584.2
Pension and other benefit obligations	100.4	101.9
Other	186.7	199.5
Total long-term liabilities and deferred credits	1,834.7	1,822.5
Total capitalization and liabilities	5,201.5	5,093.5
WPL [Member]
Utility:
Electric plant in service	3,541.6	3,481.4
Gas plant in service	429.0	424.7
Other plant in service	227.5	228.8
Accumulated depreciation	(1,422.7)	(1,372.2)
Net plant	2,775.4	2,762.7
Leased Sheboygan Falls Energy Facility, less accumulated amortization	80.1	83.2
Construction work in progress:
Edgewater Generating Station Unit 5 emission controls (Wisconsin Power and Light Company)	104.0	77.7
Columbia Energy Center Units 1 and 2 emission controls (Wisconsin Power and Light Company)	54.4	9.0
Other	75.4	73.9
Other, less accumulated depreciation	1.8	15.1
Non-regulated and other:
Total property, plant and equipment	3,091.1	3,021.6
Current assets:
Cash and cash equivalents	2.5	2.7
Accounts receivable:
Customer, less allowance for doubtful accounts	72.3	76.2
Unbilled utility revenues	76.8	75.1
Other, less allowance for doubtful accounts	37.8	38.2
Production fuel, at weighted average cost	40.8	34.2
Materials and supplies, at weighted average cost	26.6	25.7
Gas stored underground, at weighted average cost	20.1	32.2
Regulatory assets	38.8	44.6
Derivative assets	10.9	3.5
Prepaid gross receipts tax	37.3	40.2
Deferred income tax assets	43.9	6.0
Prepayments and other	25.9	7.4
Total current assets	433.7	386.0
Investments:
Investment in American Transmission Company LLC	247.0	238.8
Other	19.8	19.8
Total investments	266.8	258.6
Other assets:
Regulatory assets	330.3	333.1
Deferred charges and other	34.2	44.7
Total other assets	364.5	377.8
Total assets	4,156.1	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	517.5	507.2
Total common equity	1,452.9	1,442.4
Cumulative preferred stock (Wisconsin Power and Light Company)	60.0	60.0
Long-term debt, net (excluding current portion)	1,082.3	1,082.2
Total capitalization	2,595.2	2,584.6
Current liabilities:
Commercial paper	35.6	25.7
Accounts payable	106.4	98.5
Accounts payable to associated companies	15.0	20.5
Regulatory liabilities	49.0	27.6
Accrued interest	21.6	21.6
Derivative liabilities	25.2	31.4
Other	33.5	32.3
Total current liabilities	286.3	257.6
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	754.5	672.5
Regulatory liabilities	156.7	161.2
Capital lease obligations - Sheboygan Falls Energy Facility	101.3	103.3
Pension and other benefit obligations	127.1	128.0
Other	135.0	136.8
Total long-term liabilities and deferred credits	1,274.6	1,201.8
Total capitalization and liabilities	$ 4,156.1	$ 4,044.0