Cash, Cash Equivalents and Investments - Effective Maturity Dates of Available-for-Sale Investments (Detail)	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	$ 297,961,000
1-2 Years	241,381,000
2-3 Years	101,145,000
3-4 Years	29,807,000
4-5 Years	40,614,000
5-7 Years	10,494,000
Fair Value, Total	721,402,000
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	47,395,000
1-2 Years	154,541,000
2-3 Years	77,772,000
3-4 Years	26,545,000
4-5 Years	32,352,000
5-7 Years	4,804,000
Fair Value, Total	343,409,000
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	150,371,000
1-2 Years	23,459,000
2-3 Years	0
3-4 Years	0
4-5 Years	0
5-7 Years	0
Fair Value, Total	173,830,000
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	59,203,000
1-2 Years	46,985,000
2-3 Years	13,456,000
3-4 Years	3,262,000
4-5 Years	2,334,000
5-7 Years	5,690,000
Fair Value, Total	130,930,000
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	10,608,000
1-2 Years	8,962,000
2-3 Years	0
3-4 Years	0
4-5 Years	5,928,000
5-7 Years	0
Fair Value, Total	25,498,000
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	21,579,000
1-2 Years	4,580,000
2-3 Years	8,567,000
3-4 Years	0
4-5 Years	0
5-7 Years	0
Fair Value, Total	34,726,000
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	8,805,000
1-2 Years	2,854,000
2-3 Years	1,350,000
3-4 Years	0
4-5 Years	0
5-7 Years	0
Fair Value, Total	$ 13,009,000