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Commitments and Contingencies - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
lease
Mar. 31, 2017
USD ($)
Other Commitments [Line Items]    
Number of leases | lease 4  
Expenses incurred for the properties listed $ 0.1 $ 0.1
Line of credit    
Other Commitments [Line Items]    
Letters of credit, outstanding $ 1.0