Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2015
State Apportionment [Member]
Income Tax [Line Items]
Deferred income tax expense	$ 15
Bonus Depreciation 2014 Return [Member] | Scenario, Forecast [Member]
Income Tax [Line Items]
Deductions in income tax return		450
IPL [Member] | State Apportionment [Member]
Income Tax [Line Items]
Deferred income tax expense	8
IPL [Member] | Bonus Depreciation 2014 Return [Member] | Scenario, Forecast [Member]
Income Tax [Line Items]
Deductions in income tax return		245
WPL [Member] | State Apportionment [Member]
Income Tax [Line Items]
Deferred income tax expense	7
WPL [Member] | Bonus Depreciation 2014 Return [Member] | Scenario, Forecast [Member]
Income Tax [Line Items]
Deductions in income tax return		$ 190