Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 4.8	$ 5.8
Accounts receivable, less allowance for doubtful accounts	390.6	397.6
Production fuel, at weighted average cost	98.0	98.8
Gas stored underground, at weighted average cost	15.9	43.3
Materials and supplies, at weighted average cost	83.3	81.4
Regulatory assets	111.5	120.2
Other	64.4	79.7
Total current assets	768.5	826.8
Property, plant and equipment, net	9,626.6	9,519.1
Investments:
Investment in American Transmission Company LLC	302.5	293.3
Other	21.0	53.0
Total investments	323.5	346.3
Other assets:
Regulatory assets	1,805.7	1,788.4
Deferred charges and other	10.7	14.6
Total other assets	1,816.4	1,803.0
Total assets	12,535.0	12,495.2
Current liabilities:
Current maturities of long-term debt	313.4	313.4
Commercial paper	213.4	159.8
Accounts payable	323.8	402.4
Regulatory liabilities	193.1	187.1
Other	304.8	296.6
Total current liabilities	1,348.5	1,359.3
Long-term debt, net (excluding current portion)	3,522.7	3,522.2
Other liabilities:
Deferred tax liabilities	2,424.6	2,381.2
Regulatory liabilities	517.9	550.6
Pension and other benefit obligations	446.5	451.8
Other	310.2	306.0
Total other liabilities	$ 3,699.2	$ 3,689.6
Commitments and contingencies (Note 13)
Common equity:
Common stock	$ 1.1	$ 1.1
Additional paid-in capital	1,673.7	1,663.0
Retained earnings	2,098.9	2,068.9
Accumulated other comprehensive loss	(0.4)	(0.4)
Shares in deferred compensation trust - 214,881 and 215,093 shares at a weighted average cost of $40.36 and $39.69 per share	(8.7)	(8.5)
Total common equity	3,764.6	3,724.1
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	3,964.6	3,924.1
Total liabilities and equity	12,535.0	12,495.2
IPL [Member]
Current assets:
Cash and cash equivalents	3.0	4.5
Accounts receivable, less allowance for doubtful accounts	180.9	200.0
Production fuel, at weighted average cost	60.8	60.2
Gas stored underground, at weighted average cost	3.1	18.2
Materials and supplies, at weighted average cost	47.0	45.7
Regulatory assets	29.7	39.6
Other	13.8	28.2
Total current assets	338.3	396.4
Property, plant and equipment, net	4,993.8	4,925.1
Investments:
Total investments	0.7	19.6
Other assets:
Regulatory assets	1,376.2	1,363.0
Deferred charges and other	4.4	5.0
Total other assets	1,380.6	1,368.0
Total assets	6,713.4	6,709.1
Current liabilities:
Commercial paper	0.0
Accounts payable	153.5	197.2
Regulatory liabilities	133.1	130.9
Other	198.3	203.0
Total current liabilities	484.9	531.1
Long-term debt, net (excluding current portion)	1,857.4	1,856.9
Other liabilities:
Deferred tax liabilities	1,408.9	1,378.0
Regulatory liabilities	335.1	358.3
Pension and other benefit obligations	160.5	160.2
Other	223.7	229.3
Total other liabilities	$ 2,128.2	$ 2,125.8
Commitments and contingencies (Note 13)
Common equity:
Common stock	$ 33.4	$ 33.4
Additional paid-in capital	1,447.9	1,407.8
Retained earnings	561.6	554.1
Total common equity	2,042.9	1,995.3
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,242.9	2,195.3
Total liabilities and equity	6,713.4	6,709.1
WPL [Member]
Current assets:
Cash and cash equivalents	1.3	0.4
Accounts receivable, less allowance for doubtful accounts	192.6	185.4
Production fuel, at weighted average cost	37.2	38.6
Gas stored underground, at weighted average cost	12.8	25.1
Materials and supplies, at weighted average cost	34.0	33.5
Regulatory assets	81.8	80.6
Other	51.2	59.9
Total current assets	410.9	423.5
Property, plant and equipment, net	4,141.6	4,103.7
Investments:
Investment in American Transmission Company LLC	302.5	293.3
Other	14.7	15.4
Total investments	317.2	308.7
Other assets:
Regulatory assets	429.5	425.4
Deferred charges and other	6.7	9.1
Total other assets	436.2	434.5
Total assets	5,305.9	5,270.4
Current liabilities:
Commercial paper	25.5	19.9
Accounts payable	111.0	136.0
Regulatory liabilities	60.0	56.2
Other	146.7	124.8
Total current liabilities	343.2	336.9
Long-term debt, net (excluding current portion)	1,534.2	1,533.9
Other liabilities:
Deferred tax liabilities	1,025.1	1,005.4
Regulatory liabilities	182.8	192.3
Capital lease obligations - Sheboygan Falls Energy Facility	82.1	83.6
Pension and other benefit obligations	186.4	188.7
Other	169.0	162.0
Total other liabilities	$ 1,645.4	$ 1,632.0
Commitments and contingencies (Note 13)
Common equity:
Common stock	$ 66.2	$ 66.2
Additional paid-in capital	959.1	959.0
Retained earnings	743.8	731.1
Total common equity	1,769.1	1,756.3
Noncontrolling interest	14.0	11.3
Total equity	1,783.1	1,767.6
Total liabilities and equity	$ 5,305.9	$ 5,270.4