Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Common Equity [Line Items]
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan	7,300,000.0
Proceeds from issuance of common stock, net	$ 151.2	$ 0.0	$ 0.0
Ownership percentage	15.00%
Initial right entitlement, number of shares of common stock shareholders have the option to purchase (in shares)	0.5
Exercise price, rights (in dollars per share)	$ 110.00
Percentage of exercise price upon change in beneficial ownership	200.00%
Other comprehensive income (loss)	$ 0.2	(0.4)	0.6
IPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 554.0
Common equity ratio, computation of ratio average, number of months	13 months
Common equity ratio	42.00%
Other comprehensive income (loss)	$ 0.0	0.0	0.0
WPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 135.0
Common equity ratio, computation of ratio average, number of months	13 months
Other comprehensive income (loss)	$ 0.0	$ 0.0	$ 0.0
Minimum [Member]
Schedule of Common Equity [Line Items]
Ownership percentage	10.00%
2016 [Member] | WPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 135.0
Common equity ratio	52.25%
At The Market Offering Program [Member]
Schedule of Common Equity [Line Items]
Maximum aggregate gross sales price of common stock that can be offered and sold	$ 150.0
Common stock issued during the period, At-the-market offering program (in shares)	2,186,617	0	0
Proceeds from issuance of common stock, net	$ 133.0
Fees and commissions from issuance of common stock	$ 2.0