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Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Debt securities, Estimated Fair Value $ 182,673 $ 271,924
Equity securities, Amortized Cost 3,149  
Equity securities, Unrealized Losses (247)  
Equity securities, Estimated Fair Value 2,902  
Debt and equity securities, Amortized Cost 186,332 272,331
Debt and equity securities, Unrealized Gains 6 1
Debt and equity securities, Unrealized Losses (763) (408)
Debt and equity securities, Estimated Fair Value 185,575 271,924
U.S. Treasury notes    
Schedule of Available-for-sale Securities [Line Items]    
Debt securities, Amortized Cost   32,976
Debt securities, Unrealized Losses   (12)
Debt securities, Estimated Fair Value   32,964
Government sponsored enterprise securities    
Schedule of Available-for-sale Securities [Line Items]    
Debt securities, Amortized Cost   10,082
Debt securities, Unrealized Losses   (10)
Debt securities, Estimated Fair Value   10,072
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Debt securities, Amortized Cost 139,304 138,650
Debt securities, Unrealized Gains   1
Debt securities, Unrealized Losses (502) (321)
Debt securities, Estimated Fair Value 138,802 138,330
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Debt securities, Amortized Cost 43,879 90,623
Debt securities, Unrealized Gains 6  
Debt securities, Unrealized Losses (14) (65)
Debt securities, Estimated Fair Value $ 43,871 $ 90,558