v3.8.0.1
Employee Benefit Plans (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
year
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]        
Percentage of annual salary Contributed by the Company for employees (401(k) plans) 4.00%      
General And Administrative Expense        
Defined Benefit Plan Disclosure [Line Items]        
Defined Contribution Plan, Cost $ 74 $ 72 $ 64  
Other liabilities        
Defined Benefit Plan Disclosure [Line Items]        
Net accumulated liability for non-retiree postemployment benefits provided to former or inactive employee 0 20    
Rabbi Trust        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan Obligation 1,283 1,227    
Fair value of plan assets 881 829    
Rabbi Trust | Discontinued Operations        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan Obligation 81      
Fair value of plan assets 120 115    
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan Obligation 13,838 12,917 12,221  
Fair value of plan assets $ 13,655 $ 12,861 $ 12,541  
Defined Benefit Plan Weighted Average Asset Allocations 100.00% 100.00%    
Bond Yield rate used in determining Discount rate 3.65% 4.15% 4.50% 4.10%
Expected long-term rate of return on plan assets 6.25% 6.25% 6.25% 6.25%
Defined Benefit Plans Estimated Future Employer Contributions in Next Fiscal Year $ 200      
Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan Obligation 1,996 $ 2,084 $ 2,159  
Fair value of plan assets $ 1,615 $ 1,531 $ 1,584  
Defined Benefit Plan Weighted Average Asset Allocations 100.00% 100.00%    
Bond Yield rate used in determining Discount rate 3.60% 4.05% 4.35% 3.95%
Expected long-term rate of return on plan assets 7.00% 7.00% 7.00% 7.00%
Defined Benefit Plans Estimated Future Employer Contributions in Next Fiscal Year $ 10      
Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Bond Yield rate used in determining Discount rate 90.00%      
Maximum | Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Deferred Compensation Arrangement with Individual, Requisite Age | year 55      
Deferred Compensation Arrangement with Individual, Requisite Service Period 20 years      
Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Bond Yield rate used in determining Discount rate 10.00%      
Minimum | Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Deferred Compensation Arrangement with Individual, Requisite Age | year 50      
Deferred Compensation Arrangement with Individual, Requisite Service Period 10 years      
Annuity Contracts | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan Obligation $ 21 $ 19    
Private Equity Funds and Real Estate Funds | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Fund expected term 15 years      
Liquidation extension period (up to) 2 years      
Private Equity Funds and Real Estate Funds | Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Fund expected term 7 years      
Hedge Fund | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Lock-up restriction period 2 years      
Period for written notice 185 days      
Percent of fund's net assets 25.00%      
Hedge Fund | Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Lock-up restriction period 1 year      
Period for written notice 65 days      
Percent of fund's net assets 20.00%      
Foreign plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan Weighted Average Asset Allocations 5.00% 5.00%    
Foreign plans | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Pension benefits for foreign plans percentage of the ending benefit obligation 14.00% 14.00%