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Employee Benefit Plans (Narrative) (Details) - Supplemental Employee Retirement Plans, Defined Benefit [Member] - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]        
Net Periodic Benefit Cost $ 2 $ 4    
Accrued benefits liabilities $ 78     $ 83
Defined benefit plans and other post-retirement plans, weighted-average assumptions to determine net periodic cost, discount rate 3.40%      
Defined benefit plans and other post-retirement plans, weighted-average assumptions to determine net periodic cost, rate of compensation increase 2.00%      
Defined Benefit Plan, Noncash Settlement Loss $ 1      
Available-for-sale Securities [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Assets in a rabbi trust generally designated to pay benefits of the SERP $ 91     $ 99
Scenario, Forecast [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Noncash Settlement Loss     $ 1