Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Utility:
Electric plant	$ 10,016.3	$ 9,415.7
Gas plant	936.6	909.9
Other plant	550.4	547.9
Accumulated depreciation	(3,896.2)	(3,726.2)
Net plant	7,607.1	7,147.3
Construction work in progress	639.1	677.9
Other, less accumulated depreciation	22.0	22.3
Total utility	8,268.2	7,847.5
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	242.8	249.4
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	252.6	229.6
Total non-regulated and other	495.4	479.0
Total property, plant and equipment	8,763.6	8,326.5
Current assets:
Cash and cash equivalents	11.0	9.8
Accounts receivable, less allowance for doubtful accounts:
Customer	84.3	81.8
Unbilled utility revenues	64.6	92.3
Other	249.1	299.2
Production fuel, at weighted average cost	78.1	103.6
Materials and supplies, at weighted average cost	72.9	69.6
Gas stored underground, at weighted average cost	53.2	38.6
Regulatory assets	54.7	53.9
Other	294.4	262.4
Total current assets	962.3	1,011.2
Investments:
Investment in American Transmission Company LLC	285.6	272.1
Other	57.7	57.5
Total investments	343.3	329.6
Other assets:
Regulatory assets	1,469.6	1,359.3
Deferred charges and other	55.2	85.8
Total other assets	1,524.8	1,445.1
Total assets	11,594.0	11,112.4
Common equity:
Common stock	1.1	1.1
Additional paid-in capital	1,508.9	1,507.8
Retained earnings	1,934.5	1,780.7
Accumulated other comprehensive loss	(0.2)	(0.2)
Shares in deferred compensation trust - 234,836 and 227,469 shares at a weighted average cost of $36.94 and $35.25 per share	(8.7)	(8.0)
Total common equity	3,435.6	3,281.4
Cumulative preferred stock (Interstate Power and Light Company)	200.0	200.0
Noncontrolling interest	1.7	1.8
Total equity	3,637.3	3,483.2
Long-term debt, net (excluding current portion)	2,799.5	2,977.8
Total capitalization	6,436.8	6,461.0
Current liabilities:
Current maturities of long-term debt	492.8	358.5
Commercial paper	353.8	279.4
Accounts payable	471.1	365.0
Regulatory liabilities	220.2	196.6
Other	204.0	233.8
Total current liabilities	1,741.9	1,433.3
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	2,290.3	2,112.7
Regulatory liabilities	656.6	624.9
Pension and other benefit obligations	201.3	206.6
Other	267.1	273.9
Total long-term liabilities and deferred credits	3,415.3	3,218.1
Commitments and contingencies (Note 14)
Total capitalization and liabilities	11,594.0	11,112.4
IPL [Member]
Utility:
Electric plant	5,252.6	5,034.9
Gas plant	471.1	456.8
Other plant	306.5	302.8
Accumulated depreciation	(2,118.3)	(2,025.3)
Net plant	3,911.9	3,769.2
Construction work in progress	500.2	346.4
Other, less accumulated depreciation	21.4	21.2
Non-regulated and other:
Total property, plant and equipment	4,433.5	4,136.8
Current assets:
Cash and cash equivalents	4.6	4.4
Accounts receivable, less allowance for doubtful accounts:
Customer	200.1	246.9
Production fuel, at weighted average cost	51.8	75.6
Materials and supplies, at weighted average cost	41.3	39.4
Gas stored underground, at weighted average cost	32.5	18.9
Regulatory assets	30.3	28.5
Other	166.8	122.2
Total current assets	527.4	535.9
Investments:
Total investments	18.8	18.6
Other assets:
Regulatory assets	1,196.5	1,085.0
Deferred charges and other	18.3	29.7
Total other assets	1,214.8	1,114.7
Total assets	6,194.5	5,806.0
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,242.8	1,152.8
Retained earnings	552.8	493.5
Total common equity	1,829.0	1,679.7
Cumulative preferred stock (Interstate Power and Light Company)	200.0	200.0
Total equity	2,029.0	1,879.7
Long-term debt, net (excluding current portion)	1,370.3	1,520.0
Total capitalization	3,399.3	3,399.7
Current liabilities:
Current maturities of long-term debt	150.0	38.4
Commercial paper	38.0	0
Accounts payable	286.1	187.1
Accounts payable to associated companies	50.2	29.1
Regulatory liabilities	143.8	143.8
Accrued taxes	34.5	51.1
Other	68.4	74.8
Total current liabilities	771.0	524.3
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,316.6	1,193.0
Regulatory liabilities	479.4	471.1
Pension and other benefit obligations	46.2	48.6
Other	182.0	169.3
Total long-term liabilities and deferred credits	2,024.2	1,882.0
Commitments and contingencies (Note 14)
Total capitalization and liabilities	6,194.5	5,806.0
WPL [Member]
Utility:
Electric plant	4,763.7	4,380.8
Gas plant	465.5	453.1
Other plant	243.9	245.1
Accumulated depreciation	(1,777.9)	(1,700.9)
Net plant	3,695.2	3,378.1
Leased Sheboygan Falls Energy Facility, less accumulated amortization	66.3	70.9
Construction work in progress	138.9	331.5
Other, less accumulated depreciation	0.6	1.1
Non-regulated and other:
Total property, plant and equipment	3,901.0	3,781.6
Current assets:
Cash and cash equivalents	2.3	0.5
Accounts receivable, less allowance for doubtful accounts:
Customer	76.7	73.0
Unbilled utility revenues	64.6	92.3
Other	30.8	33.1
Production fuel, at weighted average cost	26.3	28.0
Materials and supplies, at weighted average cost	29.8	28.9
Gas stored underground, at weighted average cost	20.7	19.7
Regulatory assets	24.4	25.4
Other	113.7	101.7
Total current assets	389.3	402.6
Investments:
Investment in American Transmission Company LLC	285.6	272.1
Other	19.0	19.5
Total investments	304.6	291.6
Other assets:
Regulatory assets	273.1	274.3
Deferred charges and other	38.5	54.3
Total other assets	311.6	328.6
Total assets	4,906.5	4,804.4
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	959.0	959.0
Retained earnings	679.1	617.2
Total common equity	1,704.3	1,642.4
Noncontrolling interest	7.1	0
Total equity	1,711.4	1,642.4
Long-term debt, net (excluding current portion)	1,293.4	1,323.6
Total capitalization	3,004.8	2,966.0
Current liabilities:
Current maturities of long-term debt	30.6	8.5
Commercial paper	146.7	183.7
Accounts payable	118.1	120.0
Accounts payable to associated companies	32.4	26.0
Regulatory liabilities	76.4	52.8
Other	54.8	60.5
Total current liabilities	459.0	451.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	951.7	897.1
Regulatory liabilities	177.2	153.8
Capital lease obligations - Sheboygan Falls Energy Facility	90.7	94.5
Pension and other benefit obligations	84.3	88.4
Other	138.8	153.1
Total long-term liabilities and deferred credits	1,442.7	1,386.9
Commitments and contingencies (Note 14)
Total capitalization and liabilities	$ 4,906.5	$ 4,804.4