Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2015
2016 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] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Retail electric fuel-related costs		$ 9
2015 Test Year Retail Electric Fuel Related [Member] | WPL [Member]
Regulatory Matters [Line Items]
Annual bandwidth for fuel-related costs				2.00%
Scenario, Forecast [Member] | 2017/2018 Test Period Retail Electric [Member] | WPL [Member]
Regulatory Matters [Line Items]
Requested rate increase (decrease), amount			$ 13
Requested rate increase (decrease), percentage			1.00%
Scenario, Forecast [Member] | 2017 Test Period Retail Electric Base Rate [Member] | WPL [Member]
Regulatory Matters [Line Items]
Requested rate increase (decrease), amount			$ 65
Scenario, Forecast [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | WPL [Member]
Regulatory Matters [Line Items]
Requested rate increase (decrease), amount			(52)
Scenario, Forecast [Member] | 2017/2018 Test Period Retail Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Requested rate increase (decrease), amount			$ 9
Requested rate increase (decrease), percentage			13.00%
Scenario, Forecast [Member] | 2015 Test Year Retail Electric Fuel Related [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized refund to customers, including interest	$ 10