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Capital Management - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
CreditFacility
Dec. 31, 2017
Disclosure of objectives, policies and processes for managing capital [line items]    
Number of revolving credit facilities | CreditFacility 2  
Net debt to-capitalization ratio 0.13 0.21
Debt-plus-equity ratio 19.00% 24.00%
Debt-to-EBITDA ratio 0.9 1.1
Top of range [member]    
Disclosure of objectives, policies and processes for managing capital [line items]    
Targeted Debt to debt-plus-equity ratio 30.00%  
Targeted Debt-to-EBITDA ratio 2.5  
Net debt to-capitalization ratio 0.55  
Revolving credit facilities one [member]    
Disclosure of objectives, policies and processes for managing capital [line items]    
Borrowing capacity under revolving credit facilities $ 4,000  
Revolving credit facilities two [member]    
Disclosure of objectives, policies and processes for managing capital [line items]    
Borrowing capacity under revolving credit facilities $ 600