Condensed Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 97.6	$ 56.9
Accounts receivable, less allowance for doubtful accounts	361.6	427.3
Production fuel, at weighted average cost	67.6	83.8
Materials and supplies, at weighted average cost	81.5	72.9
Gas stored underground, at weighted average cost	18.3	67.1
Regulatory assets	59.0	68.1
Other	227.6	267.0
Total current assets	913.2	1,043.1
Property, plant and equipment, net	9,050.0	8,938.4
Investments:
Investment in American Transmission Company LLC	288.5	286.5
Other	55.4	58.4
Total investments	343.9	344.9
Other assets:
Regulatory assets	1,731.3	1,715.6
Deferred charges and other	41.2	43.9
Total other assets	1,772.5	1,759.5
Total assets	12,079.6	12,085.9
Current liabilities:
Current maturities of long-term debt	183.0	183.0
Commercial paper	41.5	141.3
Accounts payable	356.9	427.9
Regulatory liabilities	209.4	200.1
Other	232.3	262.4
Total current liabilities	1,023.1	1,214.7
Long-term debt, net (excluding current portion)	3,606.7	3,606.7
Other liabilities:
Deferred income tax liabilities	2,366.9	2,321.1
Regulatory liabilities	606.7	621.1
Pension and other benefit obligations	417.7	421.7
Other	259.7	260.1
Total other liabilities	3,651.0	3,624.0
Commitments and contingencies (Note 13)
Common equity:
Common stock	1.1	1.1
Additional paid-in capital	1,631.6	1,509.1
Retained earnings	1,973.9	1,938.0
Accumulated other comprehensive loss	(0.6)	(0.6)
Shares in deferred compensation trust - 232,550 and 238,935 shares at a weighted average cost of $38.27 and $37.45 per share	(8.9)	(8.9)
Total common equity	3,597.1	3,438.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Noncontrolling interest	1.7	1.8
Total equity	3,798.8	3,640.5
Total liabilities and equity	12,079.6	12,085.9
IPL [Member]
Current assets:
Cash and cash equivalents	2.6	5.3
Accounts receivable, less allowance for doubtful accounts	158.8	216.7
Production fuel, at weighted average cost	36.2	52.7
Materials and supplies, at weighted average cost	48.1	42.0
Gas stored underground, at weighted average cost	5.2	30.8
Regulatory assets	31.1	38.7
Other	136.5	169.9
Total current assets	418.5	556.1
Property, plant and equipment, net	4,631.4	4,554.7
Investments:
Total investments	19.1	19.1
Other assets:
Regulatory assets	1,332.6	1,319.2
Deferred charges and other	12.9	12.7
Total other assets	1,345.5	1,331.9
Total assets	6,414.5	6,461.8
Current liabilities:
Current maturities of long-term debt	150.0	150.0
Commercial paper	0
Accounts payable	199.6	259.6
Accounts payable to associated companies	34.4	31.3
Regulatory liabilities	137.5	129.7
Other	122.7	135.3
Total current liabilities	644.2	705.9
Long-term debt, net (excluding current portion)	1,618.8	1,618.7
Other liabilities:
Deferred income tax liabilities	1,370.4	1,341.4
Regulatory liabilities	434.6	453.8
Pension and other benefit obligations	142.1	142.4
Other	177.5	185.5
Total other liabilities	2,124.6	2,123.1
Commitments and contingencies (Note 13)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,242.8	1,242.8
Retained earnings	550.7	537.9
Total common equity	1,826.9	1,814.1
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,026.9	2,014.1
Total liabilities and equity	6,414.5	6,461.8
WPL [Member]
Current assets:
Cash and cash equivalents	90.9	46.7
Accounts receivable, less allowance for doubtful accounts	188.4	185.8
Production fuel, at weighted average cost	31.4	31.1
Materials and supplies, at weighted average cost	31.5	29.2
Gas stored underground, at weighted average cost	13.1	36.3
Regulatory assets	27.9	29.4
Other	83.2	98.7
Total current assets	466.4	457.2
Property, plant and equipment, net	3,966.7	3,938.9
Investments:
Investment in American Transmission Company LLC	288.5	286.5
Other	18.7	19.5
Total investments	307.2	306.0
Other assets:
Regulatory assets	398.7	396.4
Deferred charges and other	27.9	29.7
Total other assets	426.6	426.1
Total assets	5,166.9	5,128.2
Current liabilities:
Current maturities of long-term debt	30.6	30.6
Commercial paper	0
Accounts payable	107.7	112.9
Accounts payable to associated companies	21.3	25.5
Regulatory liabilities	71.9	70.4
Other	82.7	70.9
Total current liabilities	314.2	310.3
Long-term debt, net (excluding current portion)	1,543.4	1,543.3
Other liabilities:
Deferred income tax liabilities	984.4	970.0
Regulatory liabilities	172.1	167.3
Capital lease obligations - Sheboygan Falls Energy Facility	88.0	89.4
Pension and other benefit obligations	179.7	180.4
Other	159.5	155.2
Total other liabilities	1,583.7	1,562.3
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	959.0	959.0
Retained earnings	691.4	678.6
Total common equity	1,716.6	1,703.8
Noncontrolling interest	9.0	8.5
Total equity	1,725.6	1,712.3
Total liabilities and equity	$ 5,166.9	$ 5,128.2