Common Equity (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015
Common Equity [Line Items]
Proceeds from issuance of common stock, net			$ 145.4	$ 0.0
Other comprehensive income	$ 0.0	$ 0.0	0.0	0.0
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	615.0		615.0
Restricted net assets of subsidiaries	1,400.0		1,400.0
Capital contributions from parent			100.0	90.0
Common stock dividends to parent			105.0
Other comprehensive income	0.0	0.0	0.0	0.0
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	32.0		32.0
Restricted net assets of subsidiaries	1,700.0		1,700.0
Common stock dividends to parent			95.3
Other comprehensive income	$ 0.0	$ 0.0	$ 0.0	$ 0.0
At The Market Offering Program [Member]
Common Equity [Line Items]
New shares of common stock issued, At-the-market offering program (in shares)			2,186,617
Proceeds from issuance of common stock, net			$ 133.0
Fees and commissions from issuance of common stock			$ 2.0
At The Market Offering Program [Member] | Subsequent Event [Member]
Common Equity [Line Items]
Maximum aggregate gross sales price of common stock that can be offered and sold					$ 150.0