Cash, Cash Equivalents, and Investments - Effective Maturity Dates of Available-for-Sale Investments (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
1 year or less	$ 420,016
1-2 years	151,316
2-3 years	74,115
3-4 years	40,471
4-5 years	6,572
5-7 years	9,455
Fair value	701,945
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
1 year or less	138,099
1-2 years	80,605
2-3 years	68,684
3-4 years	22,429
4-5 years	1,434
5-7 years	2,855
Fair value	314,106
Treasury Bills
Schedule of Available-for-sale Securities [Line Items]
1 year or less	193,347
1-2 years	32,711
2-3 years	0
3-4 years	0
4-5 years	0
5-7 years	0
Fair value	226,058
Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
1 year or less	71,438
1-2 years	30,840
2-3 years	5,431
3-4 years	12,102
4-5 years	5,138
5-7 years	6,600
Fair value	131,549
Sovereign Bonds
Schedule of Available-for-sale Securities [Line Items]
1 year or less	8,119
1-2 years	6,318
2-3 years	0
3-4 years	0
4-5 years	0
5-7 years	0
Fair value	14,437
Agency Bonds
Schedule of Available-for-sale Securities [Line Items]
1 year or less	2,969
1-2 years	0
2-3 years	0
3-4 years	5,940
4-5 years	0
5-7 years	0
Fair value	8,909
Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
1 year or less	6,044
1-2 years	842
2-3 years	0
3-4 years	0
4-5 years	0
5-7 years	0
Fair value	$ 6,886