Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Sep. 30, 2015 USD ($)
Carryforwards [Line Items]
Net operating losses, deferred tax assets	$ 441
IPL [Member]
Carryforwards [Line Items]
Net operating losses, deferred tax assets	166
WPL [Member]
Carryforwards [Line Items]
Net operating losses, deferred tax assets	168
Federal [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	527
Net operating losses, deferred tax assets	180
Tax credits, carryforward amount	227
Tax credits, deferred tax assets	224
Federal [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	231
Net operating losses, deferred tax assets	79
Tax credits, carryforward amount	80
Tax credits, deferred tax assets	78
Federal [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	218
Net operating losses, deferred tax assets	75
Tax credits, carryforward amount	89
Tax credits, deferred tax assets	87
State [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	731
Net operating losses, deferred tax assets	37
State [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	183
Net operating losses, deferred tax assets	9
State [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	126
Net operating losses, deferred tax assets	$ 6