Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					3 Months Ended	12 Months Ended								3 Months Ended	12 Months Ended								3 Months Ended	12 Months Ended
	Dec. 31, 2012 wind_projects	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 2009 through 2011 [Member]	Dec. 31, 2012 After 2020 [Member]	Mar. 31, 2012 State Apportionment [Member]	Dec. 31, 2010 Audit Completion Income Tax Benefits [Member]	Dec. 31, 2012 Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member]	Dec. 31, 2011 Wisconsin Tax Legislation [Member]	Dec. 31, 2010 Federal Healthcare Legislation [Member]	Dec. 31, 2012 IPL [Member]	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Dec. 31, 2012 IPL [Member] After 2021 [Member]	Mar. 31, 2012 IPL [Member] State Apportionment [Member]	Dec. 31, 2010 IPL [Member] Audit Completion Income Tax Benefits [Member]	Dec. 31, 2012 IPL [Member] Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member]	Dec. 31, 2010 IPL [Member] Federal Healthcare Legislation [Member]	Dec. 31, 2012 IPL [Member] Whispering Willow - East Wind Project [Member] MW	Dec. 31, 2012 WPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Dec. 31, 2012 WPL [Member] After 2023 [Member]	Mar. 31, 2012 WPL [Member] State Apportionment [Member]	Dec. 31, 2012 WPL [Member] Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member]	Dec. 31, 2010 WPL [Member] Federal Healthcare Legislation [Member]	Dec. 31, 2012 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW	Dec. 31, 2012 WPL [Member] Cedar Ridge Wind Project [Member] MW	Dec. 31, 2012 Alliant Energy And IPL [Member]	Dec. 31, 2007 Alliant Energy And IPL [Member]	Dec. 31, 2012 Alliant Energy And IPL [Member] Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member] State Income Taxes [Member]	Dec. 31, 2011 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member] State Income Taxes [Member]	Dec. 31, 2010 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member] State Income Taxes [Member]	Dec. 31, 2012 Resources [Member] Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member]
Income Tax [Line Items]
Deferred income tax expense						$ 15				$ 7					$ 8			$ 4						$ 7		$ 3
Wind projects generating production tax credits (in wind projects)	3
Wattage capacity of projects generating production tax credits (in megawatts)																			200								200	68
Increase in income tax benefits related to change in accounting method																															13
Net income tax benefits related to tax depreciation																																3	6	12
Income tax benefit from changes in federal income tax returns							7									5
Income tax rates for continuing operations	20.80%	16.90%	32.30%				1.50%				(15.20%)	(2.70%)	22.80%			2.70%				36.30%	33.40%	39.20%
Operating loss carryforwards, utilization period									20 years
Operating losses utilized									368
State net operating loss carryforwards, deferred tax asset, valuation allowances									19
Deductions in income tax return								284									67								117										100
Taxable gain on sale of electric transmission assets																														$ 527
Gain on sale of assets, income tax obligation deferral period																													8 years
Gain on sale of assets, percent recognized during each year of deferral period	12.50%
Net operating loss carryforwards, percentage expiring					99.00%									99.00%									99.00%
Statute of limitations, expiration period from extended due date of federal tax return				3 years