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Summary of Significant Accounting Policies - Debt Issuance Costs (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]      
Debt issuance costs related to amendment $ 1,507 $ 0 $ 3,795
Capitalized debt issuance costs 12,900 11,400  
Accumulated amortization of debt issuance costs 7,400 6,000  
Amortization expense related to capitalized debt issuance costs $ 1,321 $ 1,356 $ 1,251