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Schedule of Principal Payments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
2018 $ 10,000  
2019 15,000  
2020 584,000  
2021 0  
2022 250,000  
Thereafter 600,000  
Total debt $ 1,459,000 $ 1,455,000