Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2013 wind_projects	Dec. 31, 2013 After 2024 [Member]	Mar. 31, 2012 State Apportionment [Member]	Dec. 31, 2011 Wisconsin Tax Legislation [Member]	Dec. 31, 2013 Bonus Depreciation 2013 Return [Member] Scenario, Forecast [Member]	Dec. 31, 2013 IPL [Member] After 2024 [Member]	Mar. 31, 2012 IPL [Member] State Apportionment [Member]	Dec. 31, 2013 IPL [Member] Bonus Depreciation 2013 Return [Member] Scenario, Forecast [Member]	Dec. 31, 2013 IPL [Member] Whispering Willow - East Wind Project [Member] MW	Dec. 31, 2013 WPL [Member] After 2024 [Member]	Mar. 31, 2012 WPL [Member] State Apportionment [Member]	Dec. 31, 2013 WPL [Member] Bonus Depreciation 2013 Return [Member] Scenario, Forecast [Member]	Dec. 31, 2013 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW	Dec. 31, 2013 WPL [Member] Cedar Ridge Wind Project [Member] MW
Income Tax [Line Items]
Wind projects generating production tax credits (in wind projects)	3
Wind EGU capacity (in megawatts)									200				201	68
Deferred income tax expense			$ 15				$ 8				$ 7
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					$ 130			$ 70				$ 45
Net operating loss carryforwards, percentage expiring		98.00%				95.00%				98.00%