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Fair Value Measurements - Fair Values and Unrealized Losses on Investments (Detail) - Corporate Debt [Member] - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Less than Twelve Months, Estimated fair value $ 119,485 $ 177,114
Less than Twelve Months, Gross unrealized losses (716) (536)
Twelve Months or More, Estimated fair value 60,425 26,476
Twelve Months or More, Gross unrealized losses (563) (253)
Estimated fair value, Total 179,910 203,590
Gross unrealized losses, Total $ (1,279) $ (789)