Cash, Cash Equivalents, and Investments - Gross Unrealized Losses and Fair Values for Available-for-Sale Investments (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 141,814
Unrealized losses, less than 12 months	(298)
Fair value, 12 months or greater	118,136
Unrealized losses, 12 months or greater	(407)
Total fair value	259,950
Total unrealized losses	(705)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	63,106
Unrealized losses, less than 12 months	(253)
Fair value, 12 months or greater	74,227
Unrealized losses, 12 months or greater	(252)
Total fair value	137,333
Total unrealized losses	(505)
Treasury bills
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	62,560
Unrealized losses, less than 12 months	(23)
Fair value, 12 months or greater	0
Unrealized losses, 12 months or greater	0
Total fair value	62,560
Total unrealized losses	(23)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	5,476
Unrealized losses, less than 12 months	(4)
Fair value, 12 months or greater	38,434
Unrealized losses, 12 months or greater	(135)
Total fair value	43,910
Total unrealized losses	(139)
Sovereign bonds
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	3,458
Unrealized losses, less than 12 months	0
Fair value, 12 months or greater	4,628
Unrealized losses, 12 months or greater	(12)
Total fair value	8,086
Total unrealized losses	(12)
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	1,301
Unrealized losses, less than 12 months	(1)
Fair value, 12 months or greater	847
Unrealized losses, 12 months or greater	(8)
Total fair value	2,148
Total unrealized losses	(9)
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	5,913
Unrealized losses, less than 12 months	(17)
Fair value, 12 months or greater	0
Unrealized losses, 12 months or greater	0
Total fair value	5,913
Total unrealized losses	$ (17)