Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Federal [Member]
Tax carryforwards, net operating losses	$ 852
Tax carryforwards, tax credits	267
Federal [Member] | IPL [Member]
Tax carryforwards, net operating losses	537
Tax carryforwards, tax credits	119
Federal [Member] | WPL [Member]
Tax carryforwards, net operating losses	208
Tax carryforwards, tax credits	131
State [Member]
Tax carryforwards, net operating losses	700
State [Member] | IPL [Member]
Tax carryforwards, net operating losses	13
State [Member] | WPL [Member]
Tax carryforwards, net operating losses	$ 2