Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Operating Loss Carryforwards [Line Items]
Net operating losses, deferred tax assets	$ 483
IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, deferred tax assets	193
WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, deferred tax assets	172
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	863
Net operating losses, deferred tax assets	296
Tax credits, carryforward amount	146
Tax credits, deferred tax asset	144
Federal [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	400
Net operating losses, deferred tax assets	137
Tax credits, carryforward amount	43
Tax credits, deferred tax asset	42
Federal [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	349
Net operating losses, deferred tax assets	120
Tax credits, carryforward amount	46
Tax credits, deferred tax asset	45
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	808
Net operating losses, deferred tax assets	43
State [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	238
Net operating losses, deferred tax assets	14
State [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	148
Net operating losses, deferred tax assets	$ 7