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Restricted cash - Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Cash And Cash Equivalents [Abstract]        
Cash and cash equivalents $ 154,031 $ 87,680 $ 69,786  
Restricted cash 5,968 4,757 4,940  
Restricted cash, included in other assets   $ 347    
Restricted Cash, Asset, Statement of Financial Position [Extensible List]   us-gaap:OtherAssets    
Total cash, cash equivalents, and restricted cash $ 159,999 $ 92,784 $ 74,726 $ 60,185