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Summary of Significant Accounting Policies - Summary of Available-for-sale Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 115,391 $ 104,271
Gross Unrealized Gains 1 0
Gross Unrealized Losses (1,066) (553)
Fair Value 114,326 103,718
Guaranteed Investment Certificates [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 21,708 22,261
Gross Unrealized Gains 0 0
Fair Value 21,708 22,261
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 93,683 82,010
Gross Unrealized Gains 1 0
Gross Unrealized Losses (1,066) (553)
Fair Value 92,618 $ 81,457
Less Than One Year [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 80,158  
Gross Unrealized Gains 1  
Gross Unrealized Losses (713)  
Fair Value 79,446  
Greater Than One Year [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 35,233  
Gross Unrealized Gains 0  
Gross Unrealized Losses (353)  
Fair Value $ 34,880