Income Taxes (Schedule Of Uncertain Tax Positions) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Uncertain Tax Positions [Roll Forward]
Balance, January 1	$ 0		$ 0.7		$ 23.5
Additions based on tax positions related to the current year	0		0		0.7
Reductions for tax positions of prior years	0	[1]	(0.7)	[1]	(23.5)	[1]
Balance, December 31	0		0		0.7
Statute of limitations, expiration period from extended due date of federal tax return	3 years
Statute of limitations, expiration period from extended due date of Iowa tax return	3 years
Statute of limitations, expiration period from extended due date of Wisconsin tax return	4 years
IPL [Member]
Uncertain Tax Positions [Roll Forward]
Balance, January 1	0		0		10.9
Additions based on tax positions related to the current year	0		0		0
Reductions for tax positions of prior years	0	[1]	0	[1]	(10.9)	[1]
Balance, December 31	0		0		0
WPL [Member]
Uncertain Tax Positions [Roll Forward]
Balance, January 1	0		0.7		12.6
Additions based on tax positions related to the current year	0		0		0.7
Reductions for tax positions of prior years	0	[1]	(0.7)	[1]	(12.6)	[1]
Balance, December 31	$ 0		$ 0		$ 0.7

[1] In 2012, the reductions for tax positions of prior years were due to the finalization of Alliant Energy’s federal income tax return audits for calendar years 2005 through 2009.