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Restricted Cash - Additional Information (Detail) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Restricted Cash And Cash Equivalents Items [Line Items]    
Unrestricted cash $ 29,733,000 $ 41,799,000
Certificates of deposit [Member]    
Restricted Cash And Cash Equivalents Items [Line Items]    
Security deposit 180,000 $ 260,000
Unrestricted cash $ 80,000