Condensed Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents		$ 84.7	$ 5.8
Accounts receivable, less allowance for doubtful accounts		491.5	397.6
Production fuel, at weighted average cost		92.6	98.8
Gas stored underground, at weighted average cost		37.8	43.3
Materials and supplies, at weighted average cost		89.5	81.4
Regulatory assets		63.1	120.2
Other		98.9	79.7
Total current assets		958.1	826.8
Property, plant and equipment, net		9,920.4	9,519.1
Investments:
Investment in American Transmission Company LLC		309.9	293.3
Other		19.7	53.0
Total investments		329.6	346.3
Other assets:
Regulatory assets		1,811.7	1,788.4
Deferred charges and other		9.4	14.6
Total other assets		1,821.1	1,803.0
Total assets		13,029.2	12,495.2
Current liabilities:
Current maturities of long-term debt		314.0	313.4
Commercial paper		238.3	159.8
Accounts payable		365.1	402.4
Regulatory liabilities		178.4	187.1
Other		273.9	296.6
Total current liabilities		1,369.7	1,359.3
Long-term debt, net (excluding current portion)		3,816.9	3,522.2
Other liabilities:
Deferred tax liabilities		2,530.6	2,381.2
Regulatory liabilities		497.4	550.6
Pension and other benefit obligations		455.3	451.8
Other		300.2	306.0
Total other liabilities		3,783.5	3,689.6
Commitments and contingencies (Note 13)
Common equity:
Common stock	[1]	2.3	2.3
Additional paid-in capital		1,686.0	1,661.8
Retained earnings		2,181.0	2,068.9
Accumulated other comprehensive loss		(0.4)	(0.4)
Shares in deferred compensation trust - 432,619 and 430,186 shares at a weighted average cost of $22.54 and $19.84 per share	[1]	(9.8)	(8.5)
Total common equity		3,859.1	3,724.1
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		4,059.1	3,924.1
Total liabilities and equity		13,029.2	12,495.2
IPL [Member]
Current assets:
Cash and cash equivalents		77.7	4.5
Accounts receivable, less allowance for doubtful accounts		266.8	200.0
Production fuel, at weighted average cost		70.0	60.2
Gas stored underground, at weighted average cost		17.4	18.2
Materials and supplies, at weighted average cost		50.0	45.7
Regulatory assets		15.0	39.6
Other		37.2	28.2
Total current assets		534.1	396.4
Property, plant and equipment, net		5,220.1	4,925.1
Investments:
Total investments		0.8	19.6
Other assets:
Regulatory assets		1,402.2	1,363.0
Deferred charges and other		3.8	5.0
Total other assets		1,406.0	1,368.0
Total assets		7,161.0	6,709.1
Current liabilities:
Commercial paper		0.0
Accounts payable		172.6	197.2
Accounts payable to associated companies		55.0	37.7
Regulatory liabilities		132.5	130.9
Accrued taxes		41.2	67.6
Other		85.9	97.7
Total current liabilities		487.2	531.1
Long-term debt, net (excluding current portion)		2,153.1	1,856.9
Other liabilities:
Deferred tax liabilities		1,493.6	1,378.0
Regulatory liabilities		298.9	358.3
Pension and other benefit obligations		161.2	160.2
Other		229.1	229.3
Total other liabilities		2,182.8	2,125.8
Commitments and contingencies (Note 13)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		1,472.8	1,407.8
Retained earnings		631.7	554.1
Total common equity		2,137.9	1,995.3
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		2,337.9	2,195.3
Total liabilities and equity		7,161.0	6,709.1
WPL [Member]
Current assets:
Cash and cash equivalents		5.6	0.4
Accounts receivable, less allowance for doubtful accounts		190.7	185.4
Production fuel, at weighted average cost		22.6	38.6
Gas stored underground, at weighted average cost		20.4	25.1
Materials and supplies, at weighted average cost		35.7	33.5
Regulatory assets		48.1	80.6
Other		53.9	59.9
Total current assets		377.0	423.5
Property, plant and equipment, net		4,289.1	4,103.7
Investments:
Investment in American Transmission Company LLC		309.9	293.3
Other		13.4	15.4
Total investments		323.3	308.7
Other assets:
Regulatory assets		409.5	425.4
Deferred charges and other		6.9	9.1
Total other assets		416.4	434.5
Total assets		5,405.8	5,270.4
Current liabilities:
Commercial paper		11.8	19.9
Accounts payable		122.3	136.0
Accounts payable to associated companies		32.8	21.6
Regulatory liabilities		45.9	56.2
Other		91.0	103.2
Total current liabilities		303.8	336.9
Long-term debt, net (excluding current portion)		1,534.9	1,533.9
Other liabilities:
Deferred tax liabilities		1,108.8	1,005.4
Regulatory liabilities		198.5	192.3
Capital lease obligations - Sheboygan Falls Energy Facility		78.9	83.6
Pension and other benefit obligations		186.2	188.7
Other		162.4	162.0
Total other liabilities		1,734.8	1,632.0
Commitments and contingencies (Note 13)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		959.0	959.0
Retained earnings		788.6	731.1
Total common equity		1,813.8	1,756.3
Noncontrolling interest		18.5	11.3
Total equity		1,832.3	1,767.6
Total liabilities and equity		$ 5,405.8	$ 5,270.4

[1] Share and per share amounts reflect the effects of a two-for-one common stock split distributed in May 2016.