Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Utility:
Electric plant in service	$ 8,212.9	$ 8,165.4
Gas plant in service	855.2	852.9
Other plant in service	514.4	510.1
Accumulated depreciation	(3,252.3)	(3,206.0)
Net plant	6,330.2	6,322.4
Construction work in progress:
Edgewater Generating Station Unit 5 emission controls (Wisconsin Power and Light Company)	89.0	77.7
Other	205.8	179.5
Other, less accumulated depreciation	34.7	34.9
Total utility	6,659.7	6,614.5
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	276.0	270.6
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	145.6	148.2
Total non-regulated and other	421.6	418.8
Total property, plant and equipment	7,081.3	7,033.3
Current assets:
Cash and cash equivalents	30.9	11.4
Accounts receivable:
Customer, less allowance for doubtful accounts	92.2	88.1
Unbilled utility revenues	62.2	75.1
Other, less allowance for doubtful accounts	92.7	114.9
Income tax refunds receivable	34.9	39.1
Production fuel, at weighted average cost	111.0	101.9
Materials and supplies, at weighted average cost	61.7	58.5
Gas stored underground, at weighted average cost	27.4	57.7
Regulatory assets	109.0	103.6
Prepaid gross receipts tax	31.4	40.2
Assets held for sale	66.8	119.6
Prepayments and other	60.4	60.5
Total current assets	780.6	870.6
Investments:
Investment in American Transmission Company LLC	242.3	238.8
Other	61.4	61.9
Total investments	303.7	300.7
Other assets:
Regulatory assets	1,380.4	1,391.4
Deferred charges and other	82.2	91.9
Total other assets	1,462.6	1,483.3
Total assets	9,628.2	9,687.9
Common equity:
Common stock	1.1	1.1
Additional paid-in capital	1,510.0	1,510.8
Retained earnings	1,495.2	1,510.2
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust - 249,298 and 262,735 shares at a weighted average cost of $32.10 and $31.68 per share	(8.0)	(8.3)
Total common equity	2,997.5	3,013.0
Cumulative preferred stock	145.1	145.1
Noncontrolling interest	1.8	1.8
Total equity	3,144.4	3,159.9
Cumulative preferred stock	60.0	60.0
Long-term debt, net (excluding current portion)	2,728.2	2,703.1
Total capitalization	5,932.6	5,923.0
Current liabilities:
Current maturities of long-term debt	1.4	1.4
Commercial paper	57.0	102.8
Accounts payable	263.9	267.8
Regulatory liabilities	156.4	164.7
Accrued taxes	39.7	46.9
Accrued interest	46.6	46.6
Derivative liabilities	61.9	55.9
Liabilities held for sale	59.1	62.1
Other	87.3	107.0
Total current liabilities	773.3	855.2
Other long-term liabilities and deferred credits:
Deferred income taxes	1,637.7	1,592.2
Regulatory liabilities	726.1	745.4
Pension and other benefit obligations	309.6	312.7
Other	248.9	259.4
Total long-term liabilities and deferred credits	2,922.3	2,909.7
Total capitalization and liabilities	9,628.2	9,687.9
IPL [Member]
Utility:
Electric plant in service	4,703.8	4,684.0
Gas plant in service	429.4	428.2
Steam plant in service	35.0	34.9
Other plant in service	251.5	246.4
Accumulated depreciation	(1,854.9)	(1,833.8)
Net plant	3,564.8	3,559.7
Construction work in progress:
Other	107.7	96.6
Other, less accumulated depreciation	19.9	19.8
Non-regulated and other:
Total property, plant and equipment	3,692.4	3,676.1
Current assets:
Cash and cash equivalents	23.1	2.1
Accounts receivable:
Customer, less allowance for doubtful accounts	51.9	75.2
Income tax refunds receivable	14.6	28.4
Production fuel, at weighted average cost	69.3	67.7
Materials and supplies, at weighted average cost	32.8	31.5
Gas stored underground, at weighted average cost	6.8	25.5
Regulatory assets	63.9	59.0
Prepayments and other	26.6	33.7
Total current assets	289.0	323.1
Investments:
Total investments	17.1	16.8
Other assets:
Regulatory assets	1,049.0	1,058.3
Deferred charges and other	19.1	19.2
Total other assets	1,068.1	1,077.5
Total assets	5,066.6	5,093.5
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	927.7	927.7
Retained earnings	398.9	433.3
Total common equity	1,360.0	1,394.4
Cumulative preferred stock	145.1	145.1
Total equity	1,505.1	1,539.5
Long-term debt, net (excluding current portion)	1,334.1	1,309.0
Total capitalization	2,839.2	2,848.5
Current liabilities:
Commercial paper		7.1
Accounts payable	136.7	118.2
Accounts payable to associated companies	27.3	36.7
Regulatory liabilities	117.8	137.1
Accrued taxes	48.3	43.8
Accrued interest	23.0	22.8
Derivative liabilities	30.1	24.5
Other	31.0	32.3
Total current liabilities	414.2	422.5
Other long-term liabilities and deferred credits:
Deferred income taxes	954.7	936.9
Regulatory liabilities	569.3	584.2
Pension and other benefit obligations	100.8	101.9
Other	188.4	199.5
Total long-term liabilities and deferred credits	1,813.2	1,822.5
Total capitalization and liabilities	5,066.6	5,093.5
WPL [Member]
Utility:
Electric plant in service	3,509.1	3,481.4
Gas plant in service	425.8	424.7
Other plant in service	227.9	228.8
Accumulated depreciation	(1,397.4)	(1,372.2)
Net plant	2,765.4	2,762.7
Leased Sheboygan Falls Energy Facility, less accumulated amortization	81.7	83.2
Construction work in progress:
Edgewater Generating Station Unit 5 emission controls (Wisconsin Power and Light Company)	89.0	77.7
Other	98.1	82.9
Other, less accumulated depreciation	14.8	15.1
Non-regulated and other:
Total property, plant and equipment	3,049.0	3,021.6
Current assets:
Cash and cash equivalents	2.1	2.7
Accounts receivable:
Customer, less allowance for doubtful accounts	82.6	76.2
Unbilled utility revenues	62.2	75.1
Other, less allowance for doubtful accounts	39.1	38.2
Production fuel, at weighted average cost	41.7	34.2
Materials and supplies, at weighted average cost	27.5	25.7
Gas stored underground, at weighted average cost	20.6	32.2
Regulatory assets	45.1	44.6
Prepaid gross receipts tax	31.4	40.2
Prepayments and other	27.5	16.9
Total current assets	379.8	386.0
Investments:
Investment in American Transmission Company LLC	242.3	238.8
Other	19.4	19.8
Total investments	261.7	258.6
Other assets:
Regulatory assets	331.4	333.1
Deferred charges and other	36.6	44.7
Total other assets	368.0	377.8
Total assets	4,058.5	4,044.0
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	869.1	869.0
Retained earnings	510.2	507.2
Total common equity	1,445.5	1,442.4
Cumulative preferred stock	60.0	60.0
Long-term debt, net (excluding current portion)	1,082.3	1,082.2
Total capitalization	2,587.8	2,584.6
Current liabilities:
Commercial paper	23.3	25.7
Accounts payable	85.0	98.5
Accounts payable to associated companies	19.0	20.5
Regulatory liabilities	38.6	27.6
Accrued interest	18.1	21.6
Derivative liabilities	31.8	31.4
Other	34.2	32.3
Total current liabilities	250.0	257.6
Other long-term liabilities and deferred credits:
Deferred income taxes	698.1	672.5
Regulatory liabilities	156.8	161.2
Capital lease obligations - Sheboygan Falls Energy Facility	102.3	103.3
Pension and other benefit obligations	127.6	128.0
Other	135.9	136.8
Total long-term liabilities and deferred credits	1,220.7	1,201.8
Total capitalization and liabilities	$ 4,058.5	$ 4,044.0