Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 9.2	$ 8.2
Accounts receivable, less allowance for doubtful accounts	336.1	493.3
Production fuel, at weighted average cost	80.9	98.1
Gas stored underground, at weighted average cost	40.6	37.6
Materials and supplies, at weighted average cost	99.1	86.6
Regulatory assets	84.2	57.8
Other	101.4	95.5
Total current assets	751.5	877.1
Property, plant and equipment, net	10,931.1	10,279.2
Investments:
ATC Investment	339.2	317.6
Other	119.4	20.0
Total investments	458.6	337.6
Other assets:
Regulatory assets	1,952.3	1,857.3
Deferred charges and other	21.4	22.6
Total other assets	1,973.7	1,879.9
Total assets	14,114.9	13,373.8
Current liabilities:
Current maturities of long-term debt	105.2	4.6
Commercial paper	390.3	244.1
Other short-term borrowings	95.0	0.0
Accounts payable	478.1	445.3
Regulatory liabilities	145.1	186.2
Accrued taxes	39.4	59.5
Other	217.0	222.3
Total current liabilities	1,470.1	1,162.0
Long-term debt, net (excluding current portion)	4,255.1	4,315.6
Other liabilities:
Deferred tax liabilities	2,774.7	2,570.2
Regulatory liabilities	483.4	494.8
Pension and other benefit obligations	481.3	489.9
Other	296.1	279.3
Total other liabilities	4,035.5	3,834.2
Commitments and contingencies (Note 12)
Common equity:
Common stock	2.3	2.3
Additional paid-in capital	1,838.2	1,693.1
Retained earnings	2,324.8	2,177.0
Accumulated other comprehensive loss	(0.4)	(0.4)
Shares in deferred compensation trust - 454,532 and 441,695 shares at a weighted average cost of $23.52 and $22.71 per share	(10.7)	(10.0)
Total common equity	4,154.2	3,862.0
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	4,354.2	4,062.0
Total liabilities and equity	14,114.9	13,373.8
IPL [Member]
Current assets:
Cash and cash equivalents	4.7	3.3
Accounts receivable, less allowance for doubtful accounts	143.5	240.7
Production fuel, at weighted average cost	56.7	70.3
Gas stored underground, at weighted average cost	21.6	16.3
Materials and supplies, at weighted average cost	52.6	46.5
Regulatory assets	38.9	17.7
Other	39.3	27.7
Total current assets	357.3	422.5
Property, plant and equipment, net	5,764.9	5,435.6
Other assets:
Regulatory assets	1,552.0	1,441.1
Deferred charges and other	8.5	5.5
Total other assets	1,560.5	1,446.6
Total assets	7,682.7	7,304.7
Current liabilities:
Current maturities of long-term debt	100.0	0.0
Commercial paper	4.0	0.0
Accounts payable	224.6	186.3
Accounts payable to associated companies	56.4	43.3
Regulatory liabilities	85.9	149.6
Accrued taxes	39.3	53.8
Other	92.8	88.8
Total current liabilities	603.0	521.8
Long-term debt, net (excluding current portion)	2,095.0	2,153.5
Other liabilities:
Deferred tax liabilities	1,643.5	1,511.8
Regulatory liabilities	298.9	281.2
Pension and other benefit obligations	171.4	173.2
Other	238.5	214.2
Total other liabilities	2,352.3	2,180.4
Commitments and contingencies (Note 12)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,697.8	1,597.8
Retained earnings	701.2	617.8
Total common equity	2,432.4	2,249.0
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,632.4	2,449.0
Total liabilities and equity	7,682.7	7,304.7
WPL [Member]
Current assets:
Cash and cash equivalents	3.2	4.2
Accounts receivable, less allowance for doubtful accounts	185.8	226.3
Production fuel, at weighted average cost	24.2	27.8
Gas stored underground, at weighted average cost	19.0	21.3
Materials and supplies, at weighted average cost	43.6	36.3
Regulatory assets	45.3	40.1
Other	64.6	60.5
Total current assets	385.7	416.5
Property, plant and equipment, net	4,782.4	4,426.7
Other assets:
Regulatory assets	400.3	416.2
Deferred charges and other	25.7	30.9
Total other assets	426.0	447.1
Total assets	5,594.1	5,290.3
Current liabilities:
Commercial paper	224.6	52.3
Accounts payable	197.2	192.9
Regulatory liabilities	59.2	36.6
Other	108.7	112.9
Total current liabilities	589.7	394.7
Long-term debt, net (excluding current portion)	1,536.2	1,535.2
Other liabilities:
Deferred tax liabilities	1,035.2	971.6
Regulatory liabilities	184.5	213.6
Capital lease obligations - Sheboygan Falls Energy Facility	72.0	77.2
Pension and other benefit obligations	204.2	207.8
Other	162.6	159.4
Total other liabilities	1,658.5	1,629.6
Commitments and contingencies (Note 12)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,059.0	1,019.0
Retained earnings	684.5	645.6
Total common equity	1,809.7	1,730.8
Total liabilities and equity	$ 5,594.1	$ 5,290.3