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Capital Management - Additional Information (Detail)
$ in Billions
 12 Months Ended
Dec. 31, 2017
USD ($)
CreditFacility
Dec. 31, 2016
Disclosure of objectives, policies and processes for managing capital [line items]    
Number of revolving credit facilities | CreditFacility 2  
Percentage of debt-to-capitalization covenant 50.00%  
Debt to-capitalization ratio 25.00% 32.00%
Debt-to-EBITDA ratio 1.1 2.5
Debt-to-adjusted EBITDA ratio 1.1 2.4
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  
Revolving credit facilities one [member]    
Disclosure of objectives, policies and processes for managing capital [line items]    
Borrowing capacity under revolving credit facilities $ 3.0  
Revolving credit facilities two [member]    
Disclosure of objectives, policies and processes for managing capital [line items]    
Borrowing capacity under revolving credit facilities $ 1.2