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