Income Taxes (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Income Taxes [Line Items]
Increase (decrease) in deferred tax liabilities	$ 28.8
IPL [Member]
Income Taxes [Line Items]
Increase (decrease) in deferred tax liabilities	(7.8)
WPL [Member]
Income Taxes [Line Items]
Increase (decrease) in deferred tax liabilities	$ 17.6