Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Utility:
Electric plant	$ 9,415.7	$ 9,070.7
Gas plant	909.9	878.4
Other plant	547.9	506.2
Accumulated depreciation (accum. depr.)	(3,726.2)	(3,513.0)
Net plant	7,147.3	6,942.3
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	265.0	130.4
Ottumwa Generating Station Unit 1 emission controls (IPL)	135.1	73.7
George Neal Generating Station Unit 3 emission controls (IPL)	54.6	26.5
Other	223.2	188.2
Other, less accum. depr.	22.3	21.2
Total utility	7,847.5	7,382.3
Non-regulated and other:
Non-regulated Generation, less accum. depr.	249.4	258.6
Alliant Energy Corporate Services, Inc. and other, less accum. depr.	229.6	197.1
Total non-regulated and other	479.0	455.7
Total property, plant and equipment	8,326.5	7,838.0
Current assets:
Cash and cash equivalents	9.8	21.2
Accounts receivable, less allowance for doubtul accounts:
Customer	81.8	94.9
Unbilled utility revenues	92.3	81.4
Other	299.2	209.4
Production fuel, at weighted average cost	103.6	103.1
Materials and supplies, at weighted average cost	69.6	63.1
Gas stored underground, at weighted average cost	38.6	37.7
Regulatory assets	53.9	83.5
Prepaid gross receipts tax	40.8	40.4
Deferred income tax assets	136.7	170.2
Other	84.9	89.4
Total current assets	1,011.2	994.3
Investments:
Investment in American Transmission Company LLC	272.1	257.0
Other	57.5	62.0
Total investments	329.6	319.0
Other assets:
Regulatory assets	1,359.3	1,528.9
Deferred charges and other	85.8	105.3
Total other assets	1,445.1	1,634.2
Total assets	11,112.4	10,785.5
Capitalization:
Common stock	1.1	1.1
Additional paid-in capital	1,507.8	1,511.2
Retained earnings	1,780.7	1,630.7
Accumulated other comprehensive loss	(0.2)	(0.8)
Shares in deferred compensation trust - 227,469 and 216,030 shares at a weighted average cost of $35.25 and $33.61 per share	(8.0)	(7.3)
Total common equity	3,281.4	3,134.9
Cumulative preferred stock of Interstate Power and Light Company	200.0	145.1
Noncontrolling interest	1.8	1.8
Total equity	3,483.2	3,281.8
Cumulative preferred stock of Wisconsin Power and Light Company	0	60.0
Long-term debt, net (excluding current portion)	2,977.8	3,136.6
Total capitalization	6,461.0	6,478.4
Current liabilities:
Current maturities of long-term debt	358.5	1.5
Commercial paper	279.4	217.5
Accounts payable	365.0	339.3
Regulatory liabilities	196.6	189.7
Accrued taxes	50.0	48.0
Accrued interest	50.7	48.0
Other	133.1	176.0
Total current liabilities	1,433.3	1,020.0
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	2,112.7	1,934.2
Regulatory liabilities	624.9	726.4
Pension and other benefit obligations	206.6	364.0
Other	273.9	262.5
Total long-term liabilities and deferred credits	3,218.1	3,287.1
Commitments and contingencies (Note 16)
Total capitalization and liabilities	11,112.4	10,785.5
IPL [Member]
Utility:
Electric plant	5,034.9	4,815.2
Gas plant	456.8	441.4
Other plant	302.8	289.1
Accumulated depreciation (accum. depr.)	(2,025.3)	(1,930.7)
Net plant	3,769.2	3,615.0
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	135.1	73.7
George Neal Generating Station Unit 3 emission controls (IPL)	54.6	26.5
Other	156.7	123.2
Other, less accum. depr.	21.2	19.8
Non-regulated and other:
Total property, plant and equipment	4,136.8	3,858.2
Current assets:
Cash and cash equivalents	4.4	4.5
Accounts receivable, less allowance for doubtul accounts:
Customer	246.9	95.0
Production fuel, at weighted average cost	75.6	75.2
Materials and supplies, at weighted average cost	39.4	33.3
Gas stored underground, at weighted average cost	18.9	17.2
Regulatory assets	28.5	47.6
Deferred income tax assets	87.7	79.3
Other	34.5	39.5
Total current assets	535.9	391.6
Investments:
Total investments	18.6	17.6
Other assets:
Regulatory assets	1,085.0	1,170.3
Deferred charges and other	29.7	19.3
Total other assets	1,114.7	1,189.6
Total assets	5,806.0	5,457.0
Capitalization:
Common stock	33.4	33.4
Additional paid-in capital	1,152.8	1,037.8
Retained earnings	493.5	448.0
Total common equity	1,679.7	1,519.2
Cumulative preferred stock of Interstate Power and Light Company	200.0	145.1
Total equity	1,879.7	1,664.3
Long-term debt, net (excluding current portion)	1,520.0	1,359.5
Total capitalization	3,399.7	3,023.8
Current liabilities:
Current maturities of long-term debt	38.4	0
Commercial paper	0	26.3
Accounts payable	187.1	163.2
Accounts payable to associated companies	29.1	29.3
Regulatory liabilities	143.8	130.1
Accrued taxes	51.1	46.8
Other	74.8	73.1
Total current liabilities	524.3	468.8
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,193.0	1,087.3
Regulatory liabilities	471.1	571.3
Pension and other benefit obligations	48.6	122.9
Other	169.3	182.9
Total long-term liabilities and deferred credits	1,882.0	1,964.4
Commitments and contingencies (Note 16)
Total capitalization and liabilities	5,806.0	5,457.0
WPL [Member]
Utility:
Electric plant	4,380.8	4,255.5
Gas plant	453.1	437.0
Other plant	245.1	217.1
Accumulated depreciation (accum. depr.)	(1,700.9)	(1,582.3)
Net plant	3,378.1	3,327.3
Leased Sheboygan Falls Energy Facility, less accumulated amortization of $52.9 and $46.7	70.9	77.0
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	265.0	130.4
Other	66.5	65.0
Other, less accum. depr.	1.1	1.4
Non-regulated and other:
Total property, plant and equipment	3,781.6	3,601.1
Current assets:
Cash and cash equivalents	0.5	0.7
Accounts receivable, less allowance for doubtul accounts:
Customer	73.0	83.3
Unbilled utility revenues	92.3	81.4
Other	33.1	48.5
Production fuel, at weighted average cost	28.0	27.9
Materials and supplies, at weighted average cost	28.9	28.5
Gas stored underground, at weighted average cost	19.7	20.5
Regulatory assets	25.4	35.9
Prepaid gross receipts tax	40.8	40.4
Deferred income tax assets	43.3	85.6
Other	17.6	16.0
Total current assets	402.6	468.7
Investments:
Investment in American Transmission Company LLC	272.1	257.0
Other	19.5	19.6
Total investments	291.6	276.6
Other assets:
Regulatory assets	274.3	358.6
Deferred charges and other	54.3	57.6
Total other assets	328.6	416.2
Total assets	4,804.4	4,762.6
Capitalization:
Common stock	66.2	66.2
Additional paid-in capital	959.0	959.2
Retained earnings	617.2	557.6
Total common equity	1,642.4	1,583.0
Cumulative preferred stock of Wisconsin Power and Light Company	0	60.0
Long-term debt, net (excluding current portion)	1,323.6	1,331.5
Total capitalization	2,966.0	2,974.5
Current liabilities:
Current maturities of long-term debt	8.5	0
Commercial paper	183.7	86.6
Accounts payable	120.0	126.4
Accounts payable to associated companies	26.0	13.2
Regulatory liabilities	52.8	59.6
Accrued taxes	1.4	28.3
Accrued interest	22.2	22.2
Other	36.9	49.2
Total current liabilities	451.5	385.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	897.1	844.1
Regulatory liabilities	153.8	155.1
Capital lease obligations - Sheboygan Falls Energy Facility	94.5	99.1
Pension and other benefit obligations	88.4	159.7
Other	153.1	144.6
Total long-term liabilities and deferred credits	1,386.9	1,402.6
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 4,804.4	$ 4,762.6