Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2021	Dec. 31, 2019	Dec. 31, 2017
Regulatory Matters [Line Items]
State income tax rate	11.70%	18.00%	11.80%	16.40%
Decrease in regulatory assets			$ (22.6)	$ 66.7
Regulatory assets	$ 1,678.4		1,678.4				$ 1,666.7
Regulatory liabilities	1,512.9		1,512.9				1,497.2
Refunded Federal Tax Reform benefits	$ 27.0		$ 27.0
IPL [Member]
Regulatory Matters [Line Items]
State income tax rate	4.10%	(1.30%)	5.00%	(5.30%)
Decrease in regulatory assets			$ (19.3)	$ 47.5
Increase in regulatory liabilities			19.0	$ (18.7)
Regulatory assets	$ 1,249.2		1,249.2				1,231.6
Regulatory liabilities	774.8		774.8				755.4
Refunded Federal Tax Reform benefits	$ 7.0		$ 7.0
WPL [Member]
Regulatory Matters [Line Items]
State income tax rate	13.50%	30.50%	16.00%	30.70%
Decrease in regulatory assets			$ (3.3)	$ 19.2
Regulatory assets	$ 429.2		429.2				435.1
Regulatory liabilities	738.1		738.1				741.8
Refunded Federal Tax Reform benefits	20.0		20.0
Iowa tax reform [Member] | Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Decrease in regulatory assets			33.7
Increase in regulatory liabilities			7.3
2017 Test Year Retail Gas [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase, amount	$ 11.0
Interim rate increase, percent	5.00%
2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase, amount							$ 102.0
Interim rate increase, percent							7.00%
Approved rate increase, amount			$ 130.0
Approved rate increase, percent			9.00%
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Increase in base rates	$ 5.0		$ 28.0
Scenario, Forecast [Member] | 2017 Test Year Retail Gas [Member] | IPL [Member]
Regulatory Matters [Line Items]
Requested rate increase, amount						$ 20.0
Requested rate increase, percent						8.00%
Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	10.0		10.0
Revenue Subject to Refund [Member] | IPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	7.0		7.0
Revenue Subject to Refund [Member] | WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	3.0		3.0
Deferral [Member] | 2017 and 2018 Test Period Retail Electric and Gas [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	7.0		$ 7.0
State [Member] | IPL [Member]
Regulatory Matters [Line Items]
State income tax rate			12.00%
State [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
State income tax rate					9.80%
Net Book Value of ML Kapp [Member]
Regulatory Matters [Line Items]
Regulatory assets	$ 30.0		$ 30.0