Supplemental Cash Flow Information - Cash, Cash Equalivalents and Restricted Cash (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]        
Cash and cash equivalents $ 4,653 $ 3,011    
Restricted cash [1] 4 4    
Cash, cash equivalents and restricted cash $ 4,657 [2] $ 3,015 [2] $ 2,170 $ 892
[1] The restricted cash balance is included within “Other current assets” on the consolidated balance sheets
[2] As a result of the adoption of ASU 2016-18, the consolidated statements of cash flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.”