Redeemable Preferred Stock (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Temporary Equity [Line Items]
Reduction in commercial paper	$ (138.1)	$ 11.9	$ 164.7
Preferred stock issuance costs		(5.4)
IPL [Member]
Temporary Equity [Line Items]
Cumulative preferred stock rate	5.10%
Reduction in commercial paper	0	(76.3)	69.2
Preferred stock issuance costs		(5.4)
Number of board members to be elected (in members)	2
Number of quarterly dividend requirements (in quarters)	6
IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member]
Temporary Equity [Line Items]
Preferred stock issued during period (in shares)		8,000,000
Cumulative preferred stock rate	5.10%	5.10%
Preferred stock issued during period, value		200
Preferred stock redemption price per share (in dollars per share)	$ 25
IPL [Member] | Redeemable Preferred Stock [Member] | 8.375% [Member]
Temporary Equity [Line Items]
Cumulative preferred stock rate		8.375%
Preferred stock redeemed during period (in shares)		6,000,000
Preferred stock redeemed during period, value		150
Alliant Energy and IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member]
Temporary Equity [Line Items]
Preferred stock issuance costs		5.0
Alliant Energy and IPL [Member] | Redeemable Preferred Stock [Member] | 8.375% [Member]
Temporary Equity [Line Items]
Preferred stock redemption charge		5
WPL [Member]
Temporary Equity [Line Items]
Reduction in commercial paper	(183.7)	97.1	60.9
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% through 6.50% [Member]
Temporary Equity [Line Items]
Preferred stock redeemed during period (in shares)		1,049,225
Preferred stock redeemed during period, value		61
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% [Member]
Temporary Equity [Line Items]
Cumulative preferred stock rate		4.40%
WPL [Member] | Redeemable Preferred Stock [Member] | 6.50% [Member]
Temporary Equity [Line Items]
Cumulative preferred stock rate		6.50%
Alliant Energy and WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% through 6.50% [Member]
Temporary Equity [Line Items]
Preferred stock redemption charge		1
Long-term debt [Member] | IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member]
Temporary Equity [Line Items]
Reduction in commercial paper		$ 40.0