Redeemable Preferred Stock (Narrative) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Q board_member $ / shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) shares
Temporary Equity [Line Items]
Reduction in commercial paper	$ 18.5	$ (138.1)	$ 11.9
Preferred stock issuance costs			5.4
IPL [Member]
Temporary Equity [Line Items]
Cumulative preferred stock rate	5.10%
Reduction in commercial paper	$ 0.0	0.0	(76.3)
Preferred stock issuance costs			$ 5.4
Number of board members to be elected (in members) | board_member	2
Number of quarterly dividend requirements (in quarters) | Q	6
IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member]
Temporary Equity [Line Items]
Preferred stock issued during period (in shares) | shares			8,000,000
Cumulative preferred stock rate	5.10%		5.10%
Preferred stock issued during period, value			$ 200.0
Preferred stock redemption price per share (in dollars per share) | $ / shares	$ 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) | shares			6,000,000
Preferred stock redeemed during period, value			$ 150.0
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			5.0
WPL [Member]
Temporary Equity [Line Items]
Reduction in commercial paper	$ 19.9	$ (183.7)	$ 97.1
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% through 6.50% [Member]
Temporary Equity [Line Items]
Preferred stock redeemed during period (in shares) | shares			1,049,225
Preferred stock redeemed during period, value			$ 61.0
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%
Long-term debt [Member] | IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member]
Temporary Equity [Line Items]
Reduction in commercial paper			$ 40.0