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Long-Term Debt - Additional Information 1 (Detail) - Bank Credit Facility - USD ($)
3 Months Ended
Aug. 14, 2018
Dec. 31, 2018
Sep. 30, 2018
Debt Instrument [Line Items]      
Bank credit facility $ 700,000,000   $ 700,000,000
Maturity of credit facility Aug. 14, 2023    
Bank credit facility borrowing outstanding     $ 450,000,000
Scenario, Forecast      
Debt Instrument [Line Items]      
Leverage ratio   400.00%  
Current ratio   100.00%  
Minimum      
Debt Instrument [Line Items]      
Commitment fee on unused borrowing base 0.375%    
Maximum      
Debt Instrument [Line Items]      
Commitment fee on unused borrowing base 0.50%    
LIBOR | Minimum      
Debt Instrument [Line Items]      
Spread rate for interest rate on credit facility 2.00%    
LIBOR | Maximum      
Debt Instrument [Line Items]      
Spread rate for interest rate on credit facility 3.00%    
Base Rate | Minimum      
Debt Instrument [Line Items]      
Spread rate for interest rate on credit facility 1.00%    
Base Rate | Maximum      
Debt Instrument [Line Items]      
Spread rate for interest rate on credit facility 2.00%