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Taxes - U.S. Tax Reform (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2017
Taxes      
U.S. corporate tax rate (as a percent) 21.00%    
Provisional charge from enactment of U.S. tax reform   $ 5,475 $ 5,475
Benefit for effect of tax law changes on deferred tax assets and liabilities   $ 270 $ 270
Additional provisional charge related to U.S. tax reform $ 107