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Contingent Liabilities (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jan. 17, 2014
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Loss Contingencies [Line Items]          
Legal fees   $ 4 $ 4 $ 5 $ 7
Pending Litigation | Minimum          
Loss Contingencies [Line Items]          
Loss contingency, estimate of possible loss   0   0  
Pending Litigation | Maximum          
Loss Contingencies [Line Items]          
Loss contingency, estimate of possible loss   $ 35   $ 35  
Butte Local Development v. Masters Group v. Comerica Bank          
Loss Contingencies [Line Items]          
Amount of damages awarded by the jury $ 52