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Indebtedness - Bank Debt - Additional Information (Detail) - Bank Credit Facility
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
CommercialBank
Jun. 30, 2017
Jun. 30, 2018
USD ($)
CommercialBank
Jun. 30, 2017
Debt Instrument [Line Items]        
Bank Credit facility, maximum amount $ 4,000,000,000   $ 4,000,000,000  
Bank Credit facility, initial borrowing base $ 3,000,000,000   $ 3,000,000,000  
Percentage holding of commercial banks, Maximum 5.80%   5.80%  
Number of commercial banks included in current bank group | CommercialBank 27   27  
Maturity date of loan     Apr. 13, 2023  
Bank commitments $ 2,000,000,000   $ 2,000,000,000  
Bank debt 1,300,000,000   1,300,000,000  
Undrawn letters of credit 281,400,000   281,400,000  
Borrowing capacity available under the commitment amount $ 404,600,000   $ 404,600,000  
Weighted average interest rate on the bank credit facility 3.70% 2.60% 3.50% 2.50%
Annual rate of commitment fee paid on the undrawn balance     0.35%  
Alternate Base Rate        
Debt Instrument [Line Items]        
Interest rate margin     0.75%  
LIBOR Rate        
Debt Instrument [Line Items]        
Interest rate margin     1.75%  
Minimum        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance     0.30%  
Minimum | Credit Facility        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance     0.15%  
Minimum | Alternate Base Rate        
Debt Instrument [Line Items]        
Base rate on debt 0.25%   0.25%  
Minimum | Alternate Base Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 0.125%   0.125%  
Minimum | LIBOR Rate        
Debt Instrument [Line Items]        
Base rate on debt 1.25%   1.25%  
Minimum | LIBOR Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 1.125%   1.125%  
Maximum        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance     0.375%  
Maximum | Credit Facility        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance     0.30%  
Maximum | Alternate Base Rate        
Debt Instrument [Line Items]        
Base rate on debt 1.25%   1.25%  
Maximum | Alternate Base Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 0.75%   0.75%  
Maximum | LIBOR Rate        
Debt Instrument [Line Items]        
Base rate on debt 2.25%   2.25%  
Maximum | LIBOR Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 1.75%   1.75%