Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 56.9	$ 9.8
Accounts receivable, less allowance for doubtful accounts	427.3	473.3
Production fuel, at weighted average cost	83.8	103.6
Materials and supplies, at weighted average cost	72.9	69.6
Gas stored underground, at weighted average cost	67.1	38.6
Regulatory assets	68.1	53.9
Deferred income tax assets	150.1	136.7
Other	116.9	125.7
Total current assets	1,043.1	1,011.2
Property, plant and equipment, net	8,938.4	8,326.5
Investments:
Investment in American Transmission Company LLC	286.5	272.1
Other	58.4	57.5
Total investments	344.9	329.6
Other assets:
Regulatory assets	1,715.6	1,359.3
Deferred charges and other	43.9	85.8
Total other assets	1,759.5	1,445.1
Total assets	12,085.9	11,112.4
Current liabilities:
Current maturities of long-term debt	183.0	358.5
Commercial paper	141.3	279.4
Accounts payable	427.9	365.0
Regulatory liabilities	200.1	196.6
Other	262.4	233.8
Total current liabilities	1,214.7	1,433.3
Long-term debt, net (excluding current portion)	3,606.7	2,977.8
Other liabilities:
Deferred income tax liabilities	2,321.1	2,112.7
Regulatory liabilities	621.1	624.9
Pension and other benefit obligations	421.7	206.6
Other	260.1	273.9
Total other liabilities	3,624.0	3,218.1
Commitments and contingencies (Note 16)
Equity:
Common stock	1.1	1.1
Additional paid-in capital	1,509.1	1,507.8
Retained earnings	1,938.0	1,780.7
Accumulated other comprehensive loss	(0.6)	(0.2)
Shares in deferred compensation trust - 238,935 and 227,469 shares at a weighted average cost of $37.45 and $35.25 per share	(8.9)	(8.0)
Total common equity	3,438.7	3,281.4
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Noncontrolling interest	1.8	1.8
Total equity	3,640.5	3,483.2
Total liabilities and equity	12,085.9	11,112.4
IPL [Member]
Current assets:
Cash and cash equivalents	5.3	4.4
Accounts receivable, less allowance for doubtful accounts	216.7	246.9
Production fuel, at weighted average cost	52.7	75.6
Materials and supplies, at weighted average cost	42.0	39.4
Gas stored underground, at weighted average cost	30.8	18.9
Regulatory assets	38.7	28.5
Deferred income tax assets	104.9	87.7
Other	65.0	34.5
Total current assets	556.1	535.9
Property, plant and equipment, net	4,554.7	4,136.8
Investments:
Total investments	19.1	18.6
Other assets:
Regulatory assets	1,319.2	1,085.0
Deferred charges and other	12.7	29.7
Total other assets	1,331.9	1,114.7
Total assets	6,461.8	5,806.0
Current liabilities:
Current maturities of long-term debt	150.0	38.4
Commercial paper	0	0
Accounts payable	259.6	187.1
Accounts payable to associated companies	31.3	29.1
Regulatory liabilities	129.7	143.8
Accrued taxes	45.3	51.1
Other	90.0	74.8
Total current liabilities	705.9	524.3
Long-term debt, net (excluding current portion)	1,618.7	1,520.0
Other liabilities:
Deferred income tax liabilities	1,341.4	1,193.0
Regulatory liabilities	453.8	471.1
Pension and other benefit obligations	142.4	48.6
Other	185.5	169.3
Total other liabilities	2,123.1	1,882.0
Commitments and contingencies (Note 16)
Equity:
Common stock	33.4	33.4
Additional paid-in capital	1,242.8	1,152.8
Retained earnings	537.9	493.5
Total common equity	1,814.1	1,679.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,014.1	1,879.7
Total liabilities and equity	6,461.8	5,806.0
WPL [Member]
Current assets:
Cash and cash equivalents	46.7	0.5
Accounts receivable, less allowance for doubtful accounts	185.8	198.4
Production fuel, at weighted average cost	31.1	28.0
Materials and supplies, at weighted average cost	29.2	28.9
Gas stored underground, at weighted average cost	36.3	19.7
Regulatory assets	29.4	25.4
Prepaid gross receipts tax	38.0	40.8
Deferred income tax assets	37.5	43.3
Other	23.2	17.6
Total current assets	457.2	402.6
Property, plant and equipment, net	3,938.9	3,781.6
Investments:
Investment in American Transmission Company LLC	286.5	272.1
Other	19.5	19.5
Total investments	306.0	291.6
Other assets:
Regulatory assets	396.4	274.3
Deferred charges and other	29.7	54.3
Total other assets	426.1	328.6
Total assets	5,128.2	4,804.4
Current liabilities:
Current maturities of long-term debt	30.6	8.5
Commercial paper	0	183.7
Accounts payable	112.9	120.0
Accounts payable to associated companies	25.5	26.0
Regulatory liabilities	70.4	52.8
Accrued interest	24.3	22.2
Other	46.6	38.3
Total current liabilities	310.3	451.5
Long-term debt, net (excluding current portion)	1,543.3	1,323.6
Other liabilities:
Deferred income tax liabilities	970.0	897.1
Regulatory liabilities	167.3	153.8
Capital lease obligations - Sheboygan Falls Energy Facility	89.4	94.5
Pension and other benefit obligations	180.4	88.4
Other	155.2	153.1
Total other liabilities	1,562.3	1,386.9
Commitments and contingencies (Note 16)
Equity:
Common stock	66.2	66.2
Additional paid-in capital	959.0	959.0
Retained earnings	678.6	617.2
Total common equity	1,703.8	1,642.4
Noncontrolling interest	8.5	0
Total equity	1,712.3	1,642.4
Total liabilities and equity	$ 5,128.2	$ 4,804.4