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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Income Tax Disclosure [Abstract]      
Income tax expense $ 466 $ 787  
Tax expense, entities included in effective tax rate calculation   $ 898  
Net deferred tax assets $ 22,800    
Tax expense related to tax reform legislation     $ 7,300