Common Equity (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) shares	Mar. 31, 2015 USD ($)	May. 19, 2016 shares	Apr. 20, 2016 shares	Dec. 31, 2015 shares
Common Equity [Line Items]
Common stock split conversion ratio (2:1)	2
Common stock, shares authorized (in shares) | shares	240,000,000				240,000,000
Common stock, shares outstanding (in shares) | shares	113,562,651				113,459,216
Other comprehensive income	$ 0.0	$ 0.0
IPL [Member]
Common Equity [Line Items]
Common stock, shares authorized (in shares) | shares	24,000,000				24,000,000
Common stock, shares outstanding (in shares) | shares	13,370,788				13,370,788
Retained earnings free of dividend restrictions	$ 562.0
Restricted net assets of subsidiaries	1,500.0
Capital contributions from parent	40.0	0.0
Common stock dividends to parent	38.1
Other comprehensive income	$ 0.0	0.0
WPL [Member]
Common Equity [Line Items]
Common stock, shares authorized (in shares) | shares	18,000,000				18,000,000
Common stock, shares outstanding (in shares) | shares	13,236,601				13,236,601
Retained earnings free of dividend restrictions	$ 101.0
Restricted net assets of subsidiaries	1,700.0
Common stock dividends to parent	33.8
Other comprehensive income	$ 0.0	$ 0.0
Subsequent Event [Member]
Common Equity [Line Items]
Common stock, shares authorized (in shares) | shares				480,000,000
Scenario, Forecast [Member]
Common Equity [Line Items]
Common stock, shares outstanding (in shares) | shares			227,000,000