Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018
Common Equity [Line Items]
Dividends declared per common share (in dollars per share)	$ 0.335	$ 0.315	$ 1.005	$ 0.945
Proceeds from issuance of common stock, net			$ 191.3	$ 143.2
IPL [Member]
Common Equity [Line Items]
Comprehensive income	$ 0.0	$ 0.0	0.0	0.0
WPL [Member]
Common Equity [Line Items]
Comprehensive income	$ 0.0	$ 0.0	$ 0.0	$ 0.0
At The Market Offering Program [Member]
Common Equity [Line Items]
Common stock issued during the period, At-the-market offering program (in shares)			4,171,013
Proceeds from issuance of common stock, net			$ 173.0
Fees and commissions from issuance of common stock			$ 2.0
Scenario, Forecast [Member] | At The Market Offering Program [Member]
Common Equity [Line Items]
Maximum aggregate gross sales price of common stock that can be offered and sold					$ 175.0