Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Utility:
Electric plant	$ 9,510.8	$ 9,415.7
Gas plant	918.8	909.9
Other plant	550.8	547.9
Accumulated depreciation	(3,789.5)	(3,726.2)
Net plant	7,190.9	7,147.3
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	274.6	265.0
Ottumwa Generating Station Unit 1 emission controls (IPL)	144.0	135.1
George Neal Generating Station Unit 3 emission controls (IPL)	59.6	54.6
Other	252.2	223.2
Other, less accumulated depreciation	22.3	22.3
Total utility	7,943.6	7,847.5
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	247.2	249.4
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	235.4	229.6
Total non-regulated and other	482.6	479.0
Total property, plant and equipment	8,426.2	8,326.5
Current assets:
Cash and cash equivalents	14.5	9.8
Accounts receivable, less allowance for doubtful accounts:
Customer	106.6	81.8
Unbilled utility revenues	78.3	92.3
Other	311.7	299.2
Production fuel, at weighted average cost	68.8	103.6
Materials and supplies, at weighted average cost	72.1	69.6
Gas stored underground, at weighted average cost	10.1	38.6
Regulatory assets	64.4	53.9
Other	209.7	262.4
Total current assets	936.2	1,011.2
Investments:
Investment in American Transmission Company LLC	276.9	272.1
Other	56.0	57.5
Total investments	332.9	329.6
Other assets:
Regulatory assets	1,352.9	1,359.3
Deferred charges and other	68.6	85.8
Total other assets	1,421.5	1,445.1
Total assets	11,116.8	11,112.4
Common equity:
Common stock	1.1	1.1
Additional paid-in capital	1,506.4	1,507.8
Retained earnings	1,832.2	1,780.7
Accumulated other comprehensive loss	(0.2)	(0.2)
Shares in deferred compensation trust - 224,076 and 227,469 shares at a weighted average cost of $35.89 and $35.25 per share	(8.0)	(8.0)
Total common equity	3,331.5	3,281.4
Cumulative preferred stock (Interstate Power and Light Company)	200.0	200.0
Noncontrolling interest	1.7	1.8
Total equity	3,533.2	3,483.2
Long-term debt, net (excluding current portion)	2,980.6	2,977.8
Total capitalization	6,513.8	6,461.0
Current liabilities:
Current maturities of long-term debt	358.8	358.5
Commercial paper	221.3	279.4
Accounts payable	388.1	365.0
Regulatory liabilities	224.5	196.6
Other	191.5	233.8
Total current liabilities	1,384.2	1,433.3
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	2,141.7	2,112.7
Regulatory liabilities	611.9	624.9
Pension and other benefit obligations	203.0	206.6
Other	262.2	273.9
Total long-term liabilities and deferred credits	3,218.8	3,218.1
Commitments and contingencies (Note 13)
Total capitalization and liabilities	11,116.8	11,112.4
IPL [Member]
Utility:
Electric plant	5,089.0	5,034.9
Gas plant	463.5	456.8
Other plant	304.3	302.8
Accumulated depreciation	(2,055.5)	(2,025.3)
Net plant	3,801.3	3,769.2
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	144.0	135.1
George Neal Generating Station Unit 3 emission controls (IPL)	59.6	54.6
Other	168.1	156.7
Other, less accumulated depreciation	21.3	21.2
Non-regulated and other:
Total property, plant and equipment	4,194.3	4,136.8
Current assets:
Cash and cash equivalents	5.8	4.4
Accounts receivable, less allowance for doubtful accounts:
Customer	257.5	246.9
Production fuel, at weighted average cost	54.6	75.6
Materials and supplies, at weighted average cost	40.6	39.4
Gas stored underground, at weighted average cost	5.6	18.9
Regulatory assets	26.6	28.5
Other	106.6	122.2
Total current assets	497.3	535.9
Investments:
Total investments	18.6	18.6
Other assets:
Regulatory assets	1,091.4	1,085.0
Deferred charges and other	29.0	29.7
Total other assets	1,120.4	1,114.7
Total assets	5,830.6	5,806.0
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,182.8	1,152.8
Retained earnings	501.9	493.5
Total common equity	1,718.1	1,679.7
Cumulative preferred stock (Interstate Power and Light Company)	200.0	200.0
Total equity	1,918.1	1,879.7
Long-term debt, net (excluding current portion)	1,520.2	1,520.0
Total capitalization	3,438.3	3,399.7
Current liabilities:
Current maturities of long-term debt	38.4	38.4
Commercial paper	0
Accounts payable	214.7	187.1
Accounts payable to associated companies	0.3	29.1
Regulatory liabilities	155.2	143.8
Accrued taxes	37.4	51.1
Other	61.9	74.8
Total current liabilities	507.9	524.3
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,216.0	1,193.0
Regulatory liabilities	448.8	471.1
Pension and other benefit obligations	47.9	48.6
Other	171.7	169.3
Total long-term liabilities and deferred credits	1,884.4	1,882.0
Commitments and contingencies (Note 13)
Total capitalization and liabilities	5,830.6	5,806.0
WPL [Member]
Utility:
Electric plant	4,421.8	4,380.8
Gas plant	455.3	453.1
Other plant	246.5	245.1
Accumulated depreciation	(1,734.0)	(1,700.9)
Net plant	3,389.6	3,378.1
Leased Sheboygan Falls Energy Facility, less accumulated amortization	69.3	70.9
Construction work in progress:
Columbia Energy Center Units 1 and 2 emission controls (WPL)	274.6	265.0
Other	84.1	66.5
Other, less accumulated depreciation	1.0	1.1
Non-regulated and other:
Total property, plant and equipment	3,818.6	3,781.6
Current assets:
Cash and cash equivalents	4.9	0.5
Accounts receivable, less allowance for doubtful accounts:
Customer	98.8	73.0
Unbilled utility revenues	78.3	92.3
Other	39.7	33.1
Production fuel, at weighted average cost	14.2	28.0
Materials and supplies, at weighted average cost	29.6	28.9
Gas stored underground, at weighted average cost	4.5	19.7
Regulatory assets	37.8	25.4
Other	92.5	101.7
Total current assets	400.3	402.6
Investments:
Investment in American Transmission Company LLC	276.9	272.1
Other	17.9	19.5
Total investments	294.8	291.6
Other assets:
Regulatory assets	261.5	274.3
Deferred charges and other	38.5	54.3
Total other assets	300.0	328.6
Total assets	4,813.7	4,804.4
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	959.0	959.0
Retained earnings	642.3	617.2
Total common equity	1,667.5	1,642.4
Noncontrolling interest	2.6	0
Total equity	1,670.1	1,642.4
Long-term debt, net (excluding current portion)	1,323.7	1,323.6
Total capitalization	2,993.8	2,966.0
Current liabilities:
Current maturities of long-term debt	8.5	8.5
Commercial paper	155.5	183.7
Accounts payable	114.2	120.0
Accounts payable to associated companies	20.2	26.0
Regulatory liabilities	69.3	52.8
Other	64.0	60.5
Total current liabilities	431.7	451.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	902.0	897.1
Regulatory liabilities	163.1	153.8
Capital lease obligations - Sheboygan Falls Energy Facility	93.3	94.5
Pension and other benefit obligations	87.4	88.4
Other	142.4	153.1
Total long-term liabilities and deferred credits	1,388.2	1,386.9
Commitments and contingencies (Note 13)
Total capitalization and liabilities	$ 4,813.7	$ 4,804.4