Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Mar. 31, 2017 USD ($)
Federal [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	$ 633
Tax credits, carryforward amount	284
Federal [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	315
Tax credits, carryforward amount	104
Federal [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	217
Tax credits, carryforward amount	118
State [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	696
State [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	13
State [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	$ 2