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Benefit Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Jun. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Curtailment $ 55      
Assumptions used in calculating assumed health care cost trend rate   4.00%    
Future capital call commitments for limited partnership investments   $ 384    
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments   71.00% 68.00%  
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent   50.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Match   70.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay   6.00%    
Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years   5 years    
Benefit expense for the Company's savings plan   $ 76 $ 75 $ 71
Additional Contribution, Management Discretion        
Defined Contribution Plan, Employer Matching Contribution, Percent of Match   80.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay   6.00%    
Defined Contribution Plan, Employer Discretionary Contribution Amount, Percent   2.00%    
First Year of Employment        
Defined Contribution Plan, Employer Matching Contribution, Percent of Match   35.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay   6.00%    
Pension Plans, Defined Benefit        
Settlements   $ (22) (88)  
Settlements   (22) (86)  
Defined benefit plan, accumulated benefit obligation   2,749 $ 2,729  
Future capital call commitments for limited partnership investments   $ 100    
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments   85.00% 86.00%  
Limited partnership equity related hedge fund strategy plan assets   62.00%    
Limited partnership hedge fund multi strategy plan assets   32.00%    
Limited partnership hedge fund strategy distressed investments plan assets   6.00%    
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year   $ 18    
Other Postretirement Benefits Plan        
Settlements   0 $ 0  
Settlements   $ 0 $ 0  
Defined benefit plan, health care cost trend rate assumed for next fiscal year   4.00% 4.00% 4.00%
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year   $ 2    
Minimum        
Defined benefit plan, plan assets, target allocation, percentage   40.00%    
Company Contribution of Eligible Compensation, Depending on Age, Percent   3.00%    
Maximum        
Defined benefit plan, plan assets, target allocation, percentage   60.00%    
Company Contribution of Eligible Compensation, Depending on Age, Percent   5.00%