Cash, Cash Equivalents and Investments - Components of Cash, Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Securities, Available-for-sale [Line Items]
Cash	$ 104,655	$ 97,951
Money market instruments	3,557	8,631
Cash and cash equivalents	108,212	106,582	$ 79,641	$ 51,975
Short-term investments	427,348	297,961
Long-term investments	262,039	423,441
Total	797,599	827,984
Treasury Bills [Member]
Debt Securities, Available-for-sale [Line Items]
Short-term investments	198,477	150,371
Long-term investments	32,760	23,459
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Short-term investments	137,871	47,395
Long-term investments	162,566	296,014
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Short-term investments	78,407	59,203
Long-term investments	53,631	71,727
Sovereign Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Short-term investments	8,101	21,579
Long-term investments	6,316	13,147
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Short-term investments	4,492	8,805
Long-term investments	845	4,204
Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Short-term investments	0	10,608
Long-term investments	$ 5,921	$ 14,890