Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		$ 83.8	$ 63.0	$ 67.8
Depreciation and amortization		461.8	411.6	401.3
Regulatory liabilities		1,497.2	681.0
Regulatory assets		1,666.7	1,915.1
Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		21.0	30.8
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return		56.0
Increase (decrease) in regulatory liabilities		71.2	67.3	75.5
Other operation and maintenance		403.8	383.7	389.9
Depreciation and amortization		245.0	210.8	207.2
Regulatory liabilities		755.4	430.8
Regulatory assets		1,231.6	1,458.8
IPL [Member] | Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		14.6	17.8
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return		5.0
Other operation and maintenance		249.0	219.8	235.4
Depreciation and amortization		212.9	192.5	$ 184.3
Regulatory liabilities		$ 741.8	250.2
Annual bandwidth for fuel-related costs		2.00%
Regulatory assets		$ 435.1	456.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		6.4	13.0
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		$ 17.0
Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Inflation-adjusted average of actual and estimated costs recovery period	5 years
Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] | WPL [Member]
Regulatory Matters [Line Items]
Expenditure collection period after new rate implementation		10 years
2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount		$ 102.0
Interim rate increase (decrease), percentage		7.00%
Write-down of regulatory assets		$ 9.0
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		77.0
2016 Test Year Retail Electric [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Settlement agreement rate increase, amount	$ 130.0
Settlement agreement rate increase, percentage	9.00%
2017/2018 Test Period Retail Electric [Member] | WPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		60.0
Authorized increase (decrease) in final rates, amount		9.0
Reduction in fuel-related costs		51.0
2017/2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		63.0
2017/2018 Test Period Retail Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		9.0
2017/2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		9.0
2015/2016 Test Period Retail Electric And Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		(5.0)
2015/2016 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		5.0
2016 Test Year Retail Electric Fuel Related [Member] | WPL [Member] | Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		3.0
Regulatory assets		$ 6.0
2018 Test Year Retail Electric Fuel Related [Member] | Scenario, Forecast [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount	$ 6.0
Autorized increase in final rates, percentage	1.00%
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
Forward Contracts [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	10 years
Sutherland Units One And Three [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Other operation and maintenance		$ 4.0
Sutherland Units One And Three [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	10 years
Sutherland Units One And Three [Member] | FERC [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized		10 years
AROs [Member]
Regulatory Matters [Line Items]
Regulatory assets		$ 109.3	105.9
AROs [Member] | IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		72.5	64.3
AROs [Member] | WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		36.8	41.6
AROs [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Depreciation and amortization		5.0
AROs [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	4 years
Other [Member]
Regulatory Matters [Line Items]
Regulatory assets		96.6	76.6
Other [Member] | IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		55.3	41.9
Other [Member] | WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		41.3	$ 34.7
Other [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		$ 5.0