Income Taxes (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Income Taxes [Line Items]
Increase in deferred tax liabilities	$ 33.2
IPL [Member]
Income Taxes [Line Items]
Increase in deferred tax liabilities	15.8
WPL [Member]
Income Taxes [Line Items]
Increase in deferred tax liabilities	$ 8.5