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Leases and Lease Commitments - Additional Information (Detail) - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating Leased Assets [Line Items]      
Total rental expense $ 1.7 $ 1.4 $ 1.2
Operating lease, asset $ 1.8    
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] us-gaap:OtherAssetsMember    
Operating lease, weighted average remaining lease term 7 years    
Operating lease, weighted average discount rate, percent 3.20%    
ASU No. 2016-02      
Operating Leased Assets [Line Items]      
New accounting pronouncement or change in accounting principle effect of additional assets and liabilities $ 1.3