Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents		$ 8.2	$ 5.8
Accounts receivable, less allowance for doubtful accounts		493.3	397.6
Production fuel, at weighted average cost		98.1	98.8
Gas stored underground, at weighted average cost		37.6	43.3
Materials and supplies, at weighted average cost		86.6	81.4
Regulatory assets		57.8	120.2
Other		95.5	79.7
Total current assets		877.1	826.8
Property, plant and equipment, net		10,279.2	9,519.1
Investments:
Investment in American Transmission Company LLC		317.6	293.3
Other		20.0	53.0
Total investments		337.6	346.3
Other assets:
Regulatory assets		1,857.3	1,788.4
Deferred charges and other		22.6	14.6
Total other assets		1,879.9	1,803.0
Total assets		13,373.8	12,495.2
Current liabilities:
Current maturities of long-term debt		4.6	313.4
Commercial paper		244.1	159.8
Accounts payable		445.3	402.4
Regulatory liabilities		186.2	187.1
Accrued taxes		59.5	53.2
Other		222.3	243.4
Total current liabilities		1,162.0	1,359.3
Long-term debt, net (excluding current portion)	[1]	4,315.6	3,522.2
Other liabilities:
Deferred tax liabilities		2,570.2	2,381.2
Regulatory liabilities		494.8	550.6
Pension and other benefit obligations		489.9	451.8
Other		279.3	306.0
Total other liabilities		3,834.2	3,689.6
Commitments and contingencies (Note 16)
Common equity:
Common stock	[2]	2.3	2.3
Additional paid-in capital	[2]	1,693.1	1,661.8
Retained earnings		2,177.0	2,068.9
Accumulated other comprehensive loss		(0.4)	(0.4)
Shares in deferred compensation trust - 441,695 and 430,186 shares at a weighted average cost of $22.71 and $19.84 per share	[2]	(10.0)	(8.5)
Total common equity		3,862.0	3,724.1	[3]
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		4,062.0	3,924.1
Total liabilities and equity		13,373.8	12,495.2
IPL [Member]
Current assets:
Cash and cash equivalents		3.3	4.5
Accounts receivable, less allowance for doubtful accounts		240.7	200.0
Production fuel, at weighted average cost		70.3	60.2
Gas stored underground, at weighted average cost		16.3	18.2
Materials and supplies, at weighted average cost		46.5	45.7
Regulatory assets		17.7	39.6
Other		27.7	28.2
Total current assets		422.5	396.4
Property, plant and equipment, net		5,435.6	4,925.1
Investments:
Total investments		0.8	19.6
Other assets:
Regulatory assets		1,441.1	1,363.0
Deferred charges and other		4.7	5.0
Total other assets		1,445.8	1,368.0
Total assets		7,304.7	6,709.1
Current liabilities:
Current maturities of long-term debt		0.0	0.0
Commercial paper		0.0	0.0
Accounts payable		186.3	197.2
Accounts payable to associated companies		43.3	37.7
Regulatory liabilities		149.6	130.9
Accrued taxes		53.8	67.6
Accrued interest		31.6	28.2
Other		57.2	69.5
Total current liabilities		521.8	531.1
Long-term debt, net (excluding current portion)	[1]	2,153.5	1,856.9
Other liabilities:
Deferred tax liabilities		1,511.8	1,378.0
Regulatory liabilities		281.2	358.3
Pension and other benefit obligations		173.2	160.2
Other		214.2	229.3
Total other liabilities		2,180.4	2,125.8
Commitments and contingencies (Note 16)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		1,597.8	1,407.8
Retained earnings		617.8	554.1
Total common equity		2,249.0	1,995.3
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		2,449.0	2,195.3
Total liabilities and equity		7,304.7	6,709.1
WPL [Member]
Current assets:
Cash and cash equivalents		4.2	0.4
Accounts receivable, less allowance for doubtful accounts		226.3	185.4
Production fuel, at weighted average cost		27.8	38.6
Gas stored underground, at weighted average cost		21.3	25.1
Materials and supplies, at weighted average cost		36.3	33.5
Regulatory assets		40.1	80.6
Prepaid gross receipts tax		39.8	39.2
Other		20.7	20.7
Total current assets		416.5	423.5
Property, plant and equipment, net		4,426.7	4,103.7
Investments:
Investment in American Transmission Company LLC		0.0	293.3
Other		13.7	15.4
Total investments		13.7	308.7
Other assets:
Regulatory assets		416.2	425.4
Deferred charges and other		17.2	9.1
Total other assets		433.4	434.5
Total assets		5,290.3	5,270.4
Current liabilities:
Current maturities of long-term debt		0.0	0.0
Commercial paper		52.3	19.9
Accounts payable		192.9	136.0
Accounts payable to associated companies		34.6	21.6
Regulatory liabilities		36.6	56.2
Accrued interest		23.6	23.7
Other		54.7	79.5
Total current liabilities		394.7	336.9
Long-term debt, net (excluding current portion)	[1]	1,535.2	1,533.9
Other liabilities:
Deferred tax liabilities		971.6	1,005.4
Regulatory liabilities		213.6	192.3
Capital lease obligations - Sheboygan Falls Energy Facility		77.2	83.6
Pension and other benefit obligations		207.8	188.7
Other		159.4	162.0
Total other liabilities		1,629.6	1,632.0
Commitments and contingencies (Note 16)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,019.0	959.0
Retained earnings		645.6	731.1
Total common equity		1,730.8	1,756.3
Noncontrolling interest		0.0	11.3
Total equity		1,730.8	1,767.6
Total liabilities and equity		$ 5,290.3	$ 5,270.4

[1] There were no significant sinking fund requirements related to the outstanding long-term debt.

[2] Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.

[3] Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.