Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 wind_projects	Mar. 31, 2012
Income Tax [Line Items]
Statutory federal income tax rate	35.00%
Income tax rates for continuing operations	12.70%	39.00%
Wind projects generating production tax credits (in wind projects)	3
Increase (decrease) non-current deferred tax liabilities	$ (32.8)
State Apportionment [Member]
Income Tax [Line Items]
Income tax rates for continuing operations		21.40%
Deferred income tax expense		15
Tax Benefit Rider [Member]
Income Tax [Line Items]
Income tax rates for continuing operations	12.80%	12.20%
Rate-making On Property-related Differences [Member]
Income Tax [Line Items]
Income tax rates for continuing operations	5.00%	1.70%
Deferred income tax assets [Member]
Income Tax [Line Items]
Increase (decrease) current deferred tax assets	(77.7)
IPL [Member]
Income Tax [Line Items]
Income tax rates for continuing operations	(24.00%)	125.40%
Increase (decrease) non-current deferred tax liabilities	(7.7)
IPL [Member] | State Apportionment [Member]
Income Tax [Line Items]
Income tax rates for continuing operations		137.30%
Deferred income tax expense		8
IPL [Member] | Tax Benefit Rider [Member]
Income Tax [Line Items]
Income tax rates for continuing operations	37.30%	37.50%
IPL [Member] | Rate-making On Property-related Differences [Member]
Income Tax [Line Items]
Income tax rates for continuing operations	14.00%	2.10%
IPL [Member] | Whispering Willow - East Wind Project [Member]
Income Tax [Line Items]
Electric capacity of wind projects generating production tax credits (in megawatts)	200
IPL [Member] | Deferred income tax assets [Member]
Income Tax [Line Items]
Increase (decrease) current deferred tax assets	(20.6)
WPL [Member]
Income Tax [Line Items]
Income tax rates for continuing operations	32.00%	44.10%
Increase (decrease) non-current deferred tax liabilities	(32.9)
WPL [Member] | State Apportionment [Member]
Income Tax [Line Items]
Income tax rates for continuing operations		12.30%
Deferred income tax expense		7
WPL [Member] | Cedar Ridge Wind Project [Member]
Income Tax [Line Items]
Electric capacity of wind projects generating production tax credits (in megawatts)	68
WPL [Member] | Bent Tree - Phase I Wind Project [Member]
Income Tax [Line Items]
Electric capacity of wind projects generating production tax credits (in megawatts)	200
WPL [Member] | Deferred income tax assets [Member]
Income Tax [Line Items]
Increase (decrease) current deferred tax assets	$ (63.0)