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Income Taxes - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Taxes - Narrative (Details)            
Federal statutory rate     35.00% 35.00% 35.00%  
Income Tax Expense (Benefit)     $ 8,885 $ 3,609 $ 3,553  
Other accrued liabilities   $ 3,370 3,370 2,991    
Long-term income taxes payable   6,794 6,794 1,753    
Deferred Tax Assets, Valuation Allowance   162 162 126 6  
Total Gross Unrecognized Tax Benefits   2,181 2,181 1,852 $ 1,350 $ 661
Unrecognized Tax Benefits that Would Impact Effective Tax Rate   1,800 1,800 1,800    
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued   112 112 $ 50    
Decrease in Unrecognized Tax Benefits is Reasonably Possible   800 $ 800      
Tax Cuts and Jobs Act of 2017 [Member]            
Income Taxes - Narrative (Details)            
Federal statutory rate     21.00%      
Income Tax Expense (Benefit)   5,500        
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability   308        
Taxes Payable   6,100 $ 6,100      
Other accrued liabilities   487 487      
Long-term income taxes payable   5,600 5,600      
Domestic Tax Authority [Member]            
Income Taxes - Narrative (Details)            
Operating Loss Carryforwards   1,100 1,100      
Tax Credit Carryforward, Amount   107 107      
State and Local Jurisdiction [Member]            
Income Taxes - Narrative (Details)            
Operating Loss Carryforwards   627 627      
Tax Credit Carryforward, Amount   355 $ 355      
Transition Tax [Member] | Tax Cuts and Jobs Act of 2017 [Member]            
Income Taxes - Narrative (Details)            
Income Tax Expense (Benefit)   $ 5,800        
Subsequent Event [Member]            
Income Taxes - Narrative (Details)            
Foreign Earnings Repatriated $ 28,000