Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 139.2	$ 56.9
Accounts receivable, less allowance for doubtful accounts	427.3	427.3
Production fuel, at weighted average cost	87.8	83.8
Materials and supplies, at weighted average cost	83.7	72.9
Gas stored underground, at weighted average cost	40.0	67.1
Regulatory assets	92.3	68.1
Other	217.4	267.0
Total current assets	1,087.7	1,043.1
Property, plant and equipment, net	9,366.5	8,938.4
Investments:
Investment in American Transmission Company LLC	296.7	286.5
Other	56.4	58.4
Total investments	353.1	344.9
Other assets:
Regulatory assets	1,715.6	1,715.6
Deferred charges and other	35.7	43.9
Total other assets	1,751.3	1,759.5
Total assets	12,558.6	12,085.9
Current liabilities:
Current maturities of long-term debt	3.0	183.0
Commercial paper	109.1	141.3
Accounts payable	438.3	427.9
Regulatory liabilities	189.7	200.1
Other	250.7	262.4
Total current liabilities	990.8	1,214.7
Long-term debt, net (excluding current portion)	3,855.8	3,606.7
Other liabilities:
Deferred income tax liabilities	2,462.1	2,321.1
Regulatory liabilities	554.4	621.1
Pension and other benefit obligations	416.4	421.7
Other	333.0	260.1
Total other liabilities	$ 3,765.9	$ 3,624.0
Commitments and contingencies (Note 13)
Common equity:
Common stock	$ 1.1	$ 1.1
Additional paid-in capital	1,656.0	1,509.1
Retained earnings	2,097.0	1,938.0
Accumulated other comprehensive loss	(0.6)	(0.6)
Shares in deferred compensation trust - 210,647 and 238,935 shares at a weighted average cost of $39.28 and $37.45 per share	(8.3)	(8.9)
Total common equity	3,745.2	3,438.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Noncontrolling interest	0.9	1.8
Total equity	3,946.1	3,640.5
Total liabilities and equity	12,558.6	12,085.9
IPL [Member]
Current assets:
Cash and cash equivalents	99.4	5.3
Accounts receivable, less allowance for doubtful accounts	235.7	216.7
Production fuel, at weighted average cost	51.0	52.7
Materials and supplies, at weighted average cost	46.6	42.0
Gas stored underground, at weighted average cost	20.7	30.8
Regulatory assets	33.4	38.7
Other	115.0	169.9
Total current assets	601.8	556.1
Property, plant and equipment, net	4,804.7	4,554.7
Investments:
Total investments	19.4	19.1
Other assets:
Regulatory assets	1,336.8	1,319.2
Deferred charges and other	16.8	12.7
Total other assets	1,353.6	1,331.9
Total assets	6,779.5	6,461.8
Current liabilities:
Current maturities of long-term debt	0.0	150.0
Commercial paper	0.0
Accounts payable	251.3	259.6
Accounts payable to associated companies	49.2	31.3
Regulatory liabilities	126.3	129.7
Other	125.1	135.3
Total current liabilities	551.9	705.9
Long-term debt, net (excluding current portion)	1,868.5	1,618.7
Other liabilities:
Deferred income tax liabilities	1,419.9	1,341.4
Regulatory liabilities	376.2	453.8
Pension and other benefit obligations	141.0	142.4
Other	231.0	185.5
Total other liabilities	$ 2,168.1	$ 2,123.1
Commitments and contingencies (Note 13)
Common equity:
Common stock	$ 33.4	$ 33.4
Additional paid-in capital	1,342.8	1,242.8
Retained earnings	614.8	537.9
Total common equity	1,991.0	1,814.1
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,191.0	2,014.1
Total liabilities and equity	6,779.5	6,461.8
WPL [Member]
Current assets:
Cash and cash equivalents	35.7	46.7
Accounts receivable, less allowance for doubtful accounts	173.6	185.8
Production fuel, at weighted average cost	36.8	31.1
Materials and supplies, at weighted average cost	35.3	29.2
Gas stored underground, at weighted average cost	19.3	36.3
Regulatory assets	58.9	29.4
Other	94.1	98.7
Total current assets	453.7	457.2
Property, plant and equipment, net	4,094.4	3,938.9
Investments:
Investment in American Transmission Company LLC	296.7	286.5
Other	19.1	19.5
Total investments	315.8	306.0
Other assets:
Regulatory assets	378.8	396.4
Deferred charges and other	20.3	29.7
Total other assets	399.1	426.1
Total assets	5,263.0	5,128.2
Current liabilities:
Current maturities of long-term debt	0.0	30.6
Commercial paper	0.0
Accounts payable	122.5	112.9
Accounts payable to associated companies	23.3	25.5
Regulatory liabilities	63.4	70.4
Other	86.5	70.9
Total current liabilities	295.7	310.3
Long-term debt, net (excluding current portion)	1,543.6	1,543.3
Other liabilities:
Deferred income tax liabilities	1,030.1	970.0
Regulatory liabilities	178.2	167.3
Capital lease obligations - Sheboygan Falls Energy Facility	85.1	89.4
Pension and other benefit obligations	178.2	180.4
Other	181.6	155.2
Total other liabilities	$ 1,653.2	$ 1,562.3
Commitments and contingencies (Note 13)
Common equity:
Common stock	$ 66.2	$ 66.2
Additional paid-in capital	959.0	959.0
Retained earnings	734.4	678.6
Total common equity	1,759.6	1,703.8
Noncontrolling interest	10.9	8.5
Total equity	1,770.5	1,712.3
Total liabilities and equity	$ 5,263.0	$ 5,128.2