Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Mar. 31, 2018 USD ($)
Federal [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	$ 774
Tax credits, carryforward amount	277
Federal [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	497
Tax credits, carryforward amount	123
Federal [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	176
Tax credits, carryforward amount	136
State [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	707
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