Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Mar. 31, 2016 USD ($)
Carryforwards [Line Items]
Net operating losses, deferred tax assets	$ 537
IPL [Member]
Carryforwards [Line Items]
Net operating losses, deferred tax assets	198
WPL [Member]
Carryforwards [Line Items]
Net operating losses, deferred tax assets	207
Federal [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	733
Net operating losses, deferred tax assets	252
Tax credits, carryforward amount	250
Tax credits, deferred tax assets	246
Federal [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	330
Net operating losses, deferred tax assets	111
Tax credits, carryforward amount	89
Tax credits, deferred tax assets	86
Federal [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	298
Net operating losses, deferred tax assets	104
Tax credits, carryforward amount	101
Tax credits, deferred tax assets	101
State [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	748
Net operating losses, deferred tax assets	39
State [Member] | IPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	20
Net operating losses, deferred tax assets	1
State [Member] | WPL [Member]
Carryforwards [Line Items]
Net operating losses, carryforward amount	47
Net operating losses, deferred tax assets	$ 2