Consolidated Statements Of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues:
Electric utility	$ 2,894.7	$ 2,875.5	$ 2,770.5
Gas utility	400.9	355.4	381.2
Other utility	47.5	48.6	57.9
Non-utility	39.1	40.5	44.0
Total operating revenues	3,382.2	3,320.0	3,253.6
Operating expenses:
Electric production fuel and purchased power	818.1	854.0	837.7
Electric transmission service	480.9	527.9	485.3
Cost of gas sold	211.4	194.3	219.1
Asset valuation charges for Franklin County wind farm	0.0	86.4	0.0
Other operation and maintenance	651.0	606.5	629.5
Depreciation and amortization	461.8	411.6	401.3
Taxes other than income taxes	105.6	102.3	103.7
Total operating expenses	2,728.8	2,783.0	2,676.6
Operating income (loss)	653.4	537.0	577.0
Interest expense and other:
Interest expense	215.6	196.2	187.1
Equity (income) loss from unconsolidated investments, net	(44.8)	(39.6)	(33.8)
Allowance for funds used during construction	(49.7)	(62.5)	(36.9)
Interest income and other	(0.5)	(0.5)	(0.7)
Total interest expense and other	120.6	93.6	115.7
Income from continuing operations before income taxes	532.8	443.4	461.3
Income tax expense (benefit)	66.7	59.4	70.4
Income from continuing operations, net of tax	466.1	384.0	390.9
Income (loss) from discontinued operations, net of tax	1.4	(2.3)	(2.5)
Net income	467.5	381.7	388.4
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	$ 457.3	$ 371.5	$ 378.2
Weighted average number of common shares outstanding (basic and diluted) (in shares)	229.7	227.1	225.4
Earnings per weighted average common share attributable to Alliant Energy common shareowners (basic and diluted):
Income from continuing operations, net of tax (in dollars per share)	$ 1.99	$ 1.65	$ 1.69
Loss from discontinued operations, net of tax (in dollars per share)	0.00	(0.01)	(0.01)
Net income (in dollars per share)	$ 1.99	$ 1.64	$ 1.68
Amounts attributable to common shareowners:
Income from continuing operations, net of tax	$ 455.9	$ 373.8	$ 380.7
Income (loss) from discontinued operations, net of tax	1.4	(2.3)	(2.5)
Net income	457.3	371.5	378.2
IPL [Member]
Operating revenues:
Electric utility	1,598.9	1,569.7	1,503.8
Gas utility	226.0	204.0	217.3
Other utility	45.4	46.7	53.4
Total operating revenues	1,870.3	1,820.4	1,774.5
Operating expenses:
Electric production fuel and purchased power	443.6	430.5	428.4
Electric transmission service	310.4	359.7	328.2
Cost of gas sold	115.6	111.0	123.3
Other operation and maintenance	403.8	383.7	389.9
Depreciation and amortization	245.0	210.8	207.2
Taxes other than income taxes	55.0	53.9	55.6
Total operating expenses	1,573.4	1,549.6	1,532.6
Operating income (loss)	296.9	270.8	241.9
Interest expense and other:
Interest expense	112.4	103.2	96.8
Allowance for funds used during construction	(31.4)	(52.0)	(28.2)
Interest income and other	(0.2)	(0.3)	(0.2)
Total interest expense and other	80.8	50.9	68.4
Income from continuing operations before income taxes	216.1	219.9	173.5
Income tax expense (benefit)	(10.9)	(5.9)	(22.7)
Net income	227.0	225.8	196.2
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	216.8	215.6	186.0
Amounts attributable to common shareowners:
Net income	216.8	215.6	186.0
WPL [Member]
Operating revenues:
Electric utility	1,295.8	1,305.8	1,266.7
Gas utility	174.9	151.4	163.9
Other utility	2.1	1.9	4.5
Total operating revenues	1,472.8	1,459.1	1,435.1
Operating expenses:
Electric production fuel and purchased power	374.5	423.5	409.3
Electric transmission service	170.5	168.2	157.1
Cost of gas sold	95.8	83.3	95.8
Other operation and maintenance	249.0	219.8	235.4
Depreciation and amortization	212.9	192.5	184.3
Taxes other than income taxes	46.9	44.8	44.5
Total operating expenses	1,149.6	1,132.1	1,126.4
Operating income (loss)	323.2	327.0	308.7
Interest expense and other:
Interest expense	93.8	91.4	92.4
Equity (income) loss from unconsolidated investments, net	(0.7)	(39.8)	(35.1)
Allowance for funds used during construction	(18.3)	(10.5)	(8.7)
Interest income and other	(0.1)	(0.2)	(0.4)
Total interest expense and other	74.7	40.9	48.2
Income from continuing operations before income taxes	248.5	286.1	260.5
Income tax expense (benefit)	61.9	93.3	82.9
Net income	186.6	192.8	177.6
Net income attributable to noncontrolling interest	0.0	2.4	1.3
Net income attributable to common shareowners	186.6	190.4	176.3
Amounts attributable to common shareowners:
Net income	$ 186.6	$ 190.4	$ 176.3