Assets and Liabilities Measured at Fair Value (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets
Cash equivalents		$ 25,424	$ 42,771
Total marketable securities		99,782	61,469
Municipal bonds
Fair Value Assets
Total marketable securities		46,366	32,848
Corporate bonds
Fair Value Assets
Total marketable securities		53,416	28,621
Investments in Privately-Held Companies
Fair Value Assets
Investments in privately-held companies	[1]	2,890	1,030
Level 1
Fair Value Assets
Cash equivalents		10,212	2,720
Level 2
Fair Value Assets
Cash equivalents		15,212	40,051
Total marketable securities		99,782	61,469
Level 2 | Municipal bonds
Fair Value Assets
Total marketable securities		46,366	32,848
Level 2 | Corporate bonds
Fair Value Assets
Total marketable securities		53,416	28,621
Level 3 | Investments in Privately-Held Companies
Fair Value Assets
Investments in privately-held companies	[1]	$ 2,890	$ 1,030

[1] Included in other long-term assets.