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Rate of fuel costs capacity

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Percentage of public utility's approved rate increase (decrease) by regulatory agency.

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Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested rate increase (decrease) with regulatory agency.

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Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Matters [Line Items]
Regulatory assets	$ 2,016.0	$ 2,016.0			$ 1,915.1
Regulatory liabilities	665.8	665.8			681.0
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	1,556.6	1,556.6			1,458.8
Regulatory liabilities	415.7	415.7			430.8
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	459.4	459.4			456.3
Regulatory liabilities	250.1	250.1			250.2
2016 Test Year Retail Electric [Member] \| Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Increase in electric margins	20.0	20.0
2017 and 2018 Test Period Retail Electric [Member] \| Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Increase in electric margins	16.0	38.0
2017 and 2018 Test Period Retail Gas [Member] \| Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Increase in gas margins	2.0	$ 4.0
2017 Test Year Retail Electric Fuel-related [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Annual bandwidth for fuel-related costs		2.00%
2016 Test Year Retail Electric Fuel-related [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Annual bandwidth for fuel-related costs		2.00%
Scenario, Forecast [Member] \| 2016 Test Year Retail Electric [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Requested rate increase (decrease), amount			$ 176.0
Requested rate increase (decrease), percent			12.00%
Interim rate increase (decrease), amount				$ 102.0
Interim rate increase (decrease), percent				7.00%
Scenario, Forecast [Member] \| 2017 and 2018 Test Period Retail Electric [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount				$ 9.0
Authorized increase (decrease) in final rates, percent				1.00%
Increase in base rates				$ 60.0
Reduction in fuel-related costs				51.0
Scenario, Forecast [Member] \| 2017 and 2018 Test Period Retail Gas [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount				$ 9.0
Authorized increase (decrease) in final rates, percent				13.00%
Scenario, Forecast [Member] \| 2018 Test Year Retail Electric Fuel-related [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Requested rate increase (decrease), amount			$ 6.0
Requested rate increase (decrease), percent			1.00%
Scenario, Forecast [Member] \| 2017 Test Year Retail Electric Fuel-related [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Under-collection of retail fuel-related costs				$ 12.0
FERC [Member] \| MISO Transmission Owner Complaints [Member]
Regulatory Matters [Line Items]
Base return on equity for refund period from November 12, 2013 through February 11, 2015, percent		10.32%
Refund received, after final true-up		$ 50.0
FERC [Member] \| MISO Transmission Owner Complaints [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Refund received, after final true-up		39.0
FERC [Member] \| MISO Transmission Owner Complaints [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Refund received, after final true-up		11.0
Other [Member]
Regulatory Matters [Line Items]
Regulatory assets	96.3	96.3			76.6
Other [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	52.3	52.3			41.9
Other [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	44.0	44.0			34.7
Other [Member] \| 2017 Test Year Retail Electric Fuel-related [Member] \| Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	8.0	8.0
Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	27.5	27.5			30.8
Commodity cost recovery [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	15.9	15.9			17.8
Commodity cost recovery [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	11.6	11.6			$ 13.0
Commodity cost recovery [Member] \| 2016 Test Year Retail Electric Fuel-related [Member] \| Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	$ 9.0	$ 9.0
Sutherland Units 1 And 3 [Member] \| Scenario, Forecast [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Requested recovery period				10 years