Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 5.8	$ 56.9
Accounts receivable, less allowance for doubtful accounts	397.6	427.3
Production fuel, at weighted average cost	98.8	83.8
Gas stored underground, at weighted average cost	43.3	67.1
Materials and supplies, at weighted average cost	81.4	72.9
Regulatory assets	120.2	68.1
Deferred tax assets	0.0	150.1
Other	79.7	116.9
Total current assets	826.8	1,043.1
Property, plant and equipment, net	9,519.1	8,938.4
Investments:
Investment in American Transmission Company LLC	293.3	286.5
Other	53.0	58.4
Total investments	346.3	344.9
Other assets:
Regulatory assets	1,788.4	1,715.6
Deferred charges and other	14.6	21.5
Total other assets	1,803.0	1,737.1
Total assets	12,495.2	12,063.5
Current liabilities:
Current maturities of long-term debt	313.4	183.0
Commercial paper	159.8	141.3
Accounts payable	402.4	427.9
Regulatory liabilities	187.1	200.1
Other	296.6	262.4
Total current liabilities	1,359.3	1,214.7
Long-term debt, net (excluding current portion)	3,522.2	3,584.3
Other liabilities:
Deferred tax liabilities	2,381.2	2,321.1
Regulatory liabilities	550.6	621.1
Pension and other benefit obligations	451.8	421.7
Other	306.0	260.1
Total other liabilities	$ 3,689.6	$ 3,624.0
Commitments and contingencies (Note 16)
Common equity:
Common stock	$ 1.1	$ 1.1
Additional paid-in capital	1,663.0	1,509.1
Retained earnings	2,068.9	1,938.0
Accumulated other comprehensive loss	(0.4)	(0.6)
Shares in deferred compensation trust - 215,093 and 238,935 shares at a weighted average cost of $39.69 and $37.45 per share	(8.5)	(8.9)
Total common equity	3,724.1	3,438.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Noncontrolling interest	0.0	1.8
Total equity	3,924.1	3,640.5
Total liabilities and equity	12,495.2	12,063.5
IPL [Member]
Current assets:
Cash and cash equivalents	4.5	5.3
Accounts receivable, less allowance for doubtful accounts	200.0	216.7
Production fuel, at weighted average cost	60.2	52.7
Gas stored underground, at weighted average cost	18.2	30.8
Materials and supplies, at weighted average cost	45.7	42.0
Regulatory assets	39.6	38.7
Deferred tax assets	0.0	104.0
Other	28.2	65.0
Total current assets	396.4	555.2
Property, plant and equipment, net	4,925.1	4,554.7
Investments:
Total investments	19.6	19.1
Other assets:
Regulatory assets	1,363.0	1,319.2
Deferred charges and other	5.0	2.0
Total other assets	1,368.0	1,321.2
Total assets	6,709.1	6,450.2
Current liabilities:
Current maturities of long-term debt	0.0	150.0
Commercial paper	0.0	0.0
Accounts payable	197.2	259.6
Accounts payable to associated companies	37.7	31.3
Regulatory liabilities	130.9	129.7
Accrued taxes	67.6	45.3
Accrued interest	28.2	25.4
Other	69.5	64.6
Total current liabilities	531.1	705.9
Long-term debt, net (excluding current portion)	1,856.9	1,608.0
Other liabilities:
Deferred tax liabilities	1,378.0	1,370.3
Regulatory liabilities	358.3	453.8
Pension and other benefit obligations	160.2	142.4
Other	229.3	185.5
Total other liabilities	$ 2,125.8	$ 2,152.0
Commitments and contingencies (Note 16)
Common equity:
Common stock	$ 33.4	$ 33.4
Additional paid-in capital	1,407.8	1,242.8
Retained earnings	554.1	508.1
Total common equity	1,995.3	1,784.3
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,195.3	1,984.3
Total liabilities and equity	6,709.1	6,450.2
WPL [Member]
Current assets:
Cash and cash equivalents	0.4	46.7
Accounts receivable, less allowance for doubtful accounts	185.4	185.8
Production fuel, at weighted average cost	38.6	31.1
Gas stored underground, at weighted average cost	25.1	36.3
Materials and supplies, at weighted average cost	33.5	29.2
Regulatory assets	80.6	29.4
Prepaid gross receipts tax	39.2	38.0
Deferred tax assets	0.0	37.7
Other	20.7	23.2
Total current assets	423.5	457.4
Property, plant and equipment, net	4,103.7	3,938.9
Investments:
Investment in American Transmission Company LLC	293.3	286.5
Other	15.4	19.5
Total investments	308.7	306.0
Other assets:
Regulatory assets	425.4	396.4
Deferred charges and other	9.1	18.9
Total other assets	434.5	415.3
Total assets	5,270.4	5,117.6
Current liabilities:
Current maturities of long-term debt	0.0	30.6
Commercial paper	19.9	0.0
Accounts payable	136.0	112.9
Accounts payable to associated companies	21.6	25.5
Regulatory liabilities	56.2	70.4
Accrued interest	23.7	24.3
Other	79.5	46.6
Total current liabilities	336.9	310.3
Long-term debt, net (excluding current portion)	1,533.9	1,532.5
Other liabilities:
Deferred tax liabilities	1,005.4	967.1
Regulatory liabilities	192.3	167.3
Capital lease obligations - Sheboygan Falls Energy Facility	83.6	89.4
Pension and other benefit obligations	188.7	180.4
Other	162.0	155.2
Total other liabilities	$ 1,632.0	$ 1,559.4
Commitments and contingencies (Note 16)
Common equity:
Common stock	$ 66.2	$ 66.2
Additional paid-in capital	959.0	959.0
Retained earnings	731.1	681.7
Total common equity	1,756.3	1,706.9
Noncontrolling interest	11.3	8.5
Total equity	1,767.6	1,715.4
Total liabilities and equity	$ 5,270.4	$ 5,117.6