Common Equity (Narrative) (Details) (USD $)	3 Months Ended		1 Months Ended	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015	Dec. 31, 2015
Common Equity [Line Items]
Proceeds from issuance of common stock, net	$ 122,100,000	$ 0
Other comprehensive income	0	0
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	518,000,000
Restricted net assets of subsidiaries	1,300,000,000
Common stock dividends to parent	35,000,000
Other comprehensive income	0	0
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	95,000,000
Restricted net assets of subsidiaries	1,600,000,000
Common stock dividends to parent	31,800,000
Other comprehensive income	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)	1,996,450
Proceeds from issuance of common stock, net	121,000,000
Fees and commissions from issuance of common stock	2,000,000
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				150,000,000
New shares of common stock issued, At-the-market offering program (in shares)			190,167
Proceeds from issuance of common stock, net			$ 12,000,000