Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 6.5	$ 5.8
Accounts receivable, less allowance for doubtful accounts	316.3	397.6
Production fuel, at weighted average cost	104.3	98.8
Gas stored underground, at weighted average cost	21.6	43.3
Materials and supplies, at weighted average cost	83.2	81.4
Regulatory assets	71.6	120.2
Other	98.8	79.7
Total current assets	702.3	826.8
Property, plant and equipment, net	9,846.4	9,519.1
Investments:
Investment in American Transmission Company LLC	303.5	293.3
Other	21.3	53.0
Total investments	324.8	346.3
Other assets:
Regulatory assets	1,801.0	1,788.4
Deferred charges and other	10.9	14.6
Total other assets	1,811.9	1,803.0
Total assets	12,685.4	12,495.2
Current liabilities:
Current maturities of long-term debt	314.0	313.4
Commercial paper	220.6	159.8
Accounts payable	353.0	402.4
Regulatory liabilities	201.3	187.1
Other	284.3	296.6
Total current liabilities	1,373.2	1,359.3
Long-term debt, net (excluding current portion)	3,588.4	3,522.2
Other liabilities:
Deferred tax liabilities	2,474.1	2,381.2
Regulatory liabilities	510.0	550.6
Pension and other benefit obligations	452.2	451.8
Other	297.3	306.0
Total other liabilities	3,733.6	3,689.6
Commitments and contingencies (Note 13)
Common equity:
Common stock	2.3	1.1
Additional paid-in capital	1,681.8	1,663.0
Retained earnings	2,116.1	2,068.9
Accumulated other comprehensive loss	(0.4)	(0.4)
Shares in deferred compensation trust - 430,964 and 430,186 shares at a weighted average cost of $22.25 and $19.84 per share	(9.6)	(8.5)
Total common equity	3,790.2	3,724.1
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	3,990.2	3,924.1
Total liabilities and equity	12,685.4	12,495.2
IPL [Member]
Current assets:
Cash and cash equivalents	2.4	4.5
Accounts receivable, less allowance for doubtful accounts	97.2	200.0
Production fuel, at weighted average cost	72.5	60.2
Gas stored underground, at weighted average cost	5.6	18.2
Materials and supplies, at weighted average cost	48.0	45.7
Regulatory assets	16.0	39.6
Other	33.5	28.2
Total current assets	275.2	396.4
Property, plant and equipment, net	5,127.1	4,925.1
Investments:
Total investments	0.7	19.6
Other assets:
Regulatory assets	1,389.4	1,363.0
Deferred charges and other	4.4	5.0
Total other assets	1,393.8	1,368.0
Total assets	6,796.8	6,709.1
Current liabilities:
Commercial paper	0.0
Accounts payable	160.1	197.2
Regulatory liabilities	145.3	130.9
Other	182.8	203.0
Total current liabilities	488.2	531.1
Long-term debt, net (excluding current portion)	1,924.8	1,856.9
Other liabilities:
Deferred tax liabilities	1,440.1	1,378.0
Regulatory liabilities	318.1	358.3
Pension and other benefit obligations	160.9	160.2
Other	227.9	229.3
Total other liabilities	2,147.0	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	555.5	554.1
Total common equity	2,036.8	1,995.3
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,236.8	2,195.3
Total liabilities and equity	6,796.8	6,709.1
WPL [Member]
Current assets:
Cash and cash equivalents	3.1	0.4
Accounts receivable, less allowance for doubtful accounts	185.4	185.4
Production fuel, at weighted average cost	31.8	38.6
Gas stored underground, at weighted average cost	16.0	25.1
Materials and supplies, at weighted average cost	32.6	33.5
Regulatory assets	55.6	80.6
Other	55.6	59.9
Total current assets	380.1	423.5
Property, plant and equipment, net	4,227.6	4,103.7
Investments:
Investment in American Transmission Company LLC	303.5	293.3
Other	15.1	15.4
Total investments	318.6	308.7
Other assets:
Regulatory assets	411.6	425.4
Deferred charges and other	8.5	9.1
Total other assets	420.1	434.5
Total assets	5,346.4	5,270.4
Current liabilities:
Commercial paper	45.8	19.9
Accounts payable	130.7	136.0
Regulatory liabilities	56.0	56.2
Other	120.7	124.8
Total current liabilities	353.2	336.9
Long-term debt, net (excluding current portion)	1,534.6	1,533.9
Other liabilities:
Deferred tax liabilities	1,049.5	1,005.4
Regulatory liabilities	191.9	192.3
Capital lease obligations - Sheboygan Falls Energy Facility	80.5	83.6
Pension and other benefit obligations	186.3	188.7
Other	157.2	162.0
Total other liabilities	1,665.4	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	753.3	731.1
Total common equity	1,778.6	1,756.3
Noncontrolling interest	14.6	11.3
Total equity	1,793.2	1,767.6
Total liabilities and equity	$ 5,346.4	$ 5,270.4