Employee Benefit Plans (The Estimated Portion of Balances Remaining in Accumulated Other Comprehensive Income (Loss) that are Expected to be Recognized as a Component of Net Periodic Benefit Cost) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Net loss	$ 60
Prior service credit	(27)
Qualified Plan
Net loss	51
Prior service credit	(19)
Non-Qualified Plan
Net loss	8
Prior service credit	(8)
Postretirement Benefit Plan
Net loss	1
Prior service credit	$ 0