Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Sep. 30, 2016 USD ($)
Carryforwards [Line Items]
Net operating losses, deferred tax assets	$ 496
IPL [Member]
Carryforwards [Line Items]
Net operating losses, deferred tax assets	178
WPL [Member]
Carryforwards [Line Items]
Net operating losses, deferred tax assets	192
Federal [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	587
Net operating losses, deferred tax assets	201
Tax credits, carryforward amount	264
Tax credits, deferred tax assets	260
Federal [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	255
Net operating losses, deferred tax assets	86
Tax credits, carryforward amount	95
Tax credits, deferred tax assets	91
Federal [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	242
Net operating losses, deferred tax assets	85
Tax credits, carryforward amount	108
Tax credits, deferred tax assets	107
State [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	674
Net operating losses, deferred tax assets	35
State [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	15
Net operating losses, deferred tax assets	1
State [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	3
Net operating losses, deferred tax assets	$ 0