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Indebtedness - Bank Debt - Additional Information (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
CommercialBank
Dec. 31, 2017
USD ($)
Dec. 31, 2016
Debt Instrument [Line Items]      
Bank debt $ 932,018,000 $ 1,208,467,000  
Bank Credit Facility      
Debt Instrument [Line Items]      
Bank Credit facility, maximum amount 4,000,000,000    
Bank Credit facility, borrowing base 3,000,000,000    
Bank commitments $ 2,000,000,000    
Percentage holding of commercial banks, Maximum 5.80%    
Number of commercial banks included in current bank group | CommercialBank 27    
Maturity date of loan Apr. 13, 2023    
Bank debt $ 943,000,000    
Undrawn letters of credit 281,400,000    
Borrowing capacity available under the commitment amount $ 775,600,000    
Weighted average interest rate on the bank credit facility 3.70% 2.70% 2.20%
Annual rate of commitment fee paid on the undrawn balance 0.30%    
Bank Credit Facility | Alternate Base Rate      
Debt Instrument [Line Items]      
Interest rate margin 0.50%    
Bank Credit Facility | LIBOR Rate      
Debt Instrument [Line Items]      
Interest rate margin 1.50%    
Bank Credit Facility | Minimum      
Debt Instrument [Line Items]      
Annual rate of commitment fee paid on the undrawn balance 0.30%    
Bank Credit Facility | Minimum | Credit Facility      
Debt Instrument [Line Items]      
Annual rate of commitment fee paid on the undrawn balance 0.15%    
Bank Credit Facility | Minimum | Alternate Base Rate      
Debt Instrument [Line Items]      
Base rate on debt 0.25%    
Bank Credit Facility | Minimum | Alternate Base Rate | Credit Facility      
Debt Instrument [Line Items]      
Base rate on debt 0.125%    
Bank Credit Facility | Minimum | LIBOR Rate      
Debt Instrument [Line Items]      
Base rate on debt 1.25%    
Bank Credit Facility | Minimum | LIBOR Rate | Credit Facility      
Debt Instrument [Line Items]      
Base rate on debt 1.125%    
Bank Credit Facility | Maximum      
Debt Instrument [Line Items]      
Annual rate of commitment fee paid on the undrawn balance 0.375%    
Bank Credit Facility | Maximum | Credit Facility      
Debt Instrument [Line Items]      
Annual rate of commitment fee paid on the undrawn balance 0.30%    
Bank Credit Facility | Maximum | Alternate Base Rate      
Debt Instrument [Line Items]      
Base rate on debt 1.25%    
Bank Credit Facility | Maximum | Alternate Base Rate | Credit Facility      
Debt Instrument [Line Items]      
Base rate on debt 0.75%    
Bank Credit Facility | Maximum | LIBOR Rate      
Debt Instrument [Line Items]      
Base rate on debt 2.25%    
Bank Credit Facility | Maximum | LIBOR Rate | Credit Facility      
Debt Instrument [Line Items]      
Base rate on debt 1.75%