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Accounting Standards - Summary of Adopting New Standard on the Consolidated Statement of Cash flows (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]      
Increase in changes in operating assets and liabilities $ 232 $ 39  
Decrease in cash provided from operating activities 3,329 3,266  
Reduction in purchase of subsidiaries   1,810  
Reduction in cash used for investing activities (2,128) (3,957)  
Reduction in net decrease in cash, cash equivalents and restricted cash equivalents, during the period (329) (1,695)  
Increase in cash, cash equivalents and restricted cash equivalents, end of period $ 839 1,168 $ 2,863
Accounting Standards Update 2016-18 [Member] | Restatement Adjustment [Member]      
New Accounting Pronouncement, Early Adoption [Line Items]      
Increase in changes in operating assets and liabilities   (120)  
Decrease in cash provided from operating activities   (120)  
Reduction in purchase of subsidiaries   120  
Reclassification of restricted cash deposits   194  
Reduction in cash used for investing activities   314  
Reduction in net decrease in cash, cash equivalents and restricted cash equivalents, during the period   194  
Increase in cash, cash equivalents and restricted cash equivalents, end of period   $ 194