Common Equity (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Apr. 20, 2016 shares	Dec. 31, 2015 shares
Common Equity [Line Items]
Proceeds from issuance of common stock, net			$ 13.8	$ 139.5
Common stock split conversion ratio (2:1)			2
Common stock, shares authorized (in shares) | shares	480,000,000		480,000,000		240,000,000	480,000,000
Other comprehensive income	$ 0.0	$ 0.0	$ 0.0	0.0
IPL [Member]
Common Equity [Line Items]
Common stock, shares authorized (in shares) | shares	24,000,000		24,000,000			24,000,000
Retained earnings free of dividend restrictions	$ 556.0		$ 556.0
Restricted net assets of subsidiaries	1,500.0		1,500.0
Capital contributions from parent			40.0	100.0
Common stock dividends to parent			76.1
Other comprehensive income	$ 0.0	0.0	$ 0.0	0.0
WPL [Member]
Common Equity [Line Items]
Common stock, shares authorized (in shares) | shares	18,000,000		18,000,000			18,000,000
Retained earnings free of dividend restrictions	$ 68.0		$ 68.0
Restricted net assets of subsidiaries	1,700.0		1,700.0
Common stock dividends to parent			67.5
Other 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) | shares				4,373,234
Proceeds from issuance of common stock, net				$ 133.0
Fees and commissions from issuance of common stock				$ 2.0