Cash, Cash Equivalents and Investments - Effective Maturity Dates of Available-for-Sale Investments (Detail)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	$ 427,348,000
1-2 Years	149,507,000
2-3 Years	64,174,000
3-4 Years	36,985,000
4-5 Years	7,542,000
5-7 Years	3,831,000
Fair Value, Total	689,387,000
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	137,871,000
1-2 Years	81,921,000
2-3 Years	56,459,000
3-4 Years	20,060,000
4-5 Years	4,126,000
5-7 Years	0
Fair Value, Total	300,437,000
Treasury Bills [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	198,477,000
1-2 Years	32,760,000
2-3 Years	0
3-4 Years	0
4-5 Years	0
5-7 Years	0
Fair Value, Total	231,237,000
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	78,407,000
1-2 Years	27,665,000
2-3 Years	7,715,000
3-4 Years	11,004,000
4-5 Years	3,416,000
5-7 Years	3,831,000
Fair Value, Total	132,038,000
Sovereign Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	8,101,000
1-2 Years	6,316,000
2-3 Years	0
3-4 Years	0
4-5 Years	0
5-7 Years	0
Fair Value, Total	14,417,000
Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	0
1-2 Years	0
2-3 Years	0
3-4 Years	5,921,000
4-5 Years	0
5-7 Years	0
Fair Value, Total	5,921,000
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	4,492,000
1-2 Years	845,000
2-3 Years	0
3-4 Years	0
4-5 Years	0
5-7 Years	0
Fair Value, Total	$ 5,337,000