Cash, Cash Equivalents, and Investments - Effective Maturity Dates of Available-for-Sale Investments (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
1 year or less	$ 441,337
1-2 years	231,056
2-3 years	50,840
3-4 years	26,040
4-5 years	6,252
5-7 years	2,866
Fair value	758,391
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
1 year or less	137,228
1-2 years	102,129
2-3 years	35,329
3-4 years	12,360
4-5 years	2,827
5-7 years	0
Fair value	289,873
Treasury bills
Debt Securities, Available-for-sale [Line Items]
1 year or less	189,808
1-2 years	105,298
2-3 years	0
3-4 years	0
4-5 years	0
5-7 years	0
Fair value	295,106
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
1 year or less	98,910
1-2 years	19,283
2-3 years	8,119
3-4 years	7,767
4-5 years	3,425
5-7 years	2,866
Fair value	140,370
Sovereign bonds
Debt Securities, Available-for-sale [Line Items]
1 year or less	10,900
1-2 years	3,499
2-3 years	7,392
3-4 years	0
4-5 years	0
5-7 years	0
Fair value	21,791
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
1 year or less	4,491
1-2 years	847
2-3 years	0
3-4 years	0
4-5 years	0
5-7 years	0
Fair value	5,338
Agency bonds
Debt Securities, Available-for-sale [Line Items]
1 year or less	0
1-2 years	0
2-3 years	0
3-4 years	5,913
4-5 years	0
5-7 years	0
Fair value	$ 5,913