Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Utility:
Electric plant	$ 9,803.9	$ 9,415.7
Gas plant	926.8	909.9
Other plant	551.0	547.9
Accumulated depreciation	(3,838.8)	(3,726.2)
Net plant	7,442.9	7,147.3
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	160.3	135.1
Other	452.9	542.8
Other, less accumulated depreciation	21.9	22.3
Total utility	8,078.0	7,847.5
Non-regulated and other:
Non-regulated Generation, less accumulated depreciation	244.9	249.4
Alliant Energy Corporate Services, Inc. and other, less accumulated depreciation	243.3	229.6
Total non-regulated and other	488.2	479.0
Total property, plant and equipment	8,566.2	8,326.5
Current assets:
Cash and cash equivalents	16.2	9.8
Accounts receivable, less allowance for doubtful accounts:
Customer	85.9	81.8
Unbilled utility revenues	77.5	92.3
Other	318.6	299.2
Production fuel, at weighted average cost	74.6	103.6
Materials and supplies, at weighted average cost	75.5	69.6
Gas stored underground, at weighted average cost	22.7	38.6
Regulatory assets	49.4	53.9
Other	289.7	262.4
Total current assets	1,010.1	1,011.2
Investments:
Investment in American Transmission Company LLC	281.6	272.1
Other	57.0	57.5
Total investments	338.6	329.6
Other assets:
Regulatory assets	1,403.8	1,359.3
Deferred charges and other	67.9	85.8
Total other assets	1,471.7	1,445.1
Total assets	11,386.6	11,112.4
Common equity:
Common stock	1.1	1.1
Additional paid-in capital	1,507.3	1,507.8
Retained earnings	1,837.6	1,780.7
Accumulated other comprehensive loss	(0.2)	(0.2)
Shares in deferred compensation trust - 229,489 and 227,469 shares at a weighted average cost of $36.44 and $35.25 per share	(8.4)	(8.0)
Total common equity	3,337.4	3,281.4
Cumulative preferred stock (Interstate Power and Light Company)	200.0	200.0
Noncontrolling interest	1.7	1.8
Total equity	3,539.1	3,483.2
Long-term debt, net (excluding current portion)	2,829.9	2,977.8
Total capitalization	6,369.0	6,461.0
Current liabilities:
Current maturities of long-term debt	509.0	358.5
Commercial paper	307.9	279.4
Accounts payable	475.6	365.0
Regulatory liabilities	224.8	196.6
Other	215.9	233.8
Total current liabilities	1,733.2	1,433.3
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	2,189.9	2,112.7
Regulatory liabilities	638.7	624.9
Pension and other benefit obligations	202.1	206.6
Other	253.7	273.9
Total long-term liabilities and deferred credits	3,284.4	3,218.1
Commitments and contingencies (Note 14)
Total capitalization and liabilities	11,386.6	11,112.4
IPL [Member]
Utility:
Electric plant	5,200.1	5,034.9
Gas plant	467.2	456.8
Other plant	305.3	302.8
Accumulated depreciation	(2,087.5)	(2,025.3)
Net plant	3,885.1	3,769.2
Construction work in progress:
Ottumwa Generating Station Unit 1 emission controls (IPL)	160.3	135.1
Other	212.6	211.3
Other, less accumulated depreciation	21.3	21.2
Non-regulated and other:
Total property, plant and equipment	4,279.3	4,136.8
Current assets:
Cash and cash equivalents	5.4	4.4
Accounts receivable, less allowance for doubtful accounts:
Customer	242.8	246.9
Production fuel, at weighted average cost	53.0	75.6
Materials and supplies, at weighted average cost	43.1	39.4
Gas stored underground, at weighted average cost	11.4	18.9
Regulatory assets	22.8	28.5
Other	168.1	122.2
Total current assets	546.6	535.9
Investments:
Total investments	18.9	18.6
Other assets:
Regulatory assets	1,129.8	1,085.0
Deferred charges and other	26.3	29.7
Total other assets	1,156.1	1,114.7
Total assets	6,000.9	5,806.0
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,212.8	1,152.8
Retained earnings	485.3	493.5
Total common equity	1,731.5	1,679.7
Cumulative preferred stock (Interstate Power and Light Company)	200.0	200.0
Total equity	1,931.5	1,879.7
Long-term debt, net (excluding current portion)	1,370.3	1,520.0
Total capitalization	3,301.8	3,399.7
Current liabilities:
Current maturities of long-term debt	188.4	38.4
Commercial paper	0
Accounts payable	262.9	187.1
Accounts payable to associated companies	48.0	29.1
Regulatory liabilities	152.5	143.8
Other	119.9	125.9
Total current liabilities	771.7	524.3
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	1,250.0	1,193.0
Regulatory liabilities	456.1	471.1
Pension and other benefit obligations	47.0	48.6
Other	174.3	169.3
Total long-term liabilities and deferred credits	1,927.4	1,882.0
Commitments and contingencies (Note 14)
Total capitalization and liabilities	6,000.9	5,806.0
WPL [Member]
Utility:
Electric plant	4,603.8	4,380.8
Gas plant	459.6	453.1
Other plant	245.7	245.1
Accumulated depreciation	(1,751.3)	(1,700.9)
Net plant	3,557.8	3,378.1
Leased Sheboygan Falls Energy Facility, less accumulated amortization	67.8	70.9
Construction work in progress:
Other	240.3	331.5
Other, less accumulated depreciation	0.6	1.1
Non-regulated and other:
Total property, plant and equipment	3,866.5	3,781.6
Current assets:
Cash and cash equivalents	6.4	0.5
Accounts receivable, less allowance for doubtful accounts:
Customer	77.4	73.0
Unbilled utility revenues	77.5	92.3
Other	41.4	33.1
Production fuel, at weighted average cost	21.6	28.0
Materials and supplies, at weighted average cost	30.1	28.9
Gas stored underground, at weighted average cost	11.3	19.7
Regulatory assets	26.6	25.4
Other	109.6	101.7
Total current assets	401.9	402.6
Investments:
Investment in American Transmission Company LLC	281.6	272.1
Other	18.4	19.5
Total investments	300.0	291.6
Other assets:
Regulatory assets	274.0	274.3
Deferred charges and other	42.6	54.3
Total other assets	316.6	328.6
Total assets	4,885.0	4,804.4
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	959.0	959.0
Retained earnings	647.2	617.2
Total common equity	1,672.4	1,642.4
Noncontrolling interest	5.2	0
Total equity	1,677.6	1,642.4
Long-term debt, net (excluding current portion)	1,323.8	1,323.6
Total capitalization	3,001.4	2,966.0
Current liabilities:
Current maturities of long-term debt	8.5	8.5
Commercial paper	158.0	183.7
Accounts payable	152.1	120.0
Accounts payable to associated companies	21.5	26.0
Regulatory liabilities	72.3	52.8
Other	60.0	60.5
Total current liabilities	472.4	451.5
Other long-term liabilities and deferred credits:
Deferred income tax liabilities	918.2	897.1
Regulatory liabilities	182.6	153.8
Capital lease obligations - Sheboygan Falls Energy Facility	92.0	94.5
Pension and other benefit obligations	85.2	88.4
Other	133.2	153.1
Total long-term liabilities and deferred credits	1,411.2	1,386.9
Commitments and contingencies (Note 14)
Total capitalization and liabilities	$ 4,885.0	$ 4,804.4