Schedule of Unrealized Loss on Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	$ 20,924	$ 42,906
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	22	207
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	15,526	3,681
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	75	57
Available-for-sale investments with unrealized losses, Fair Value, Total	36,450	46,587
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	97	264
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value		1,363
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss		6
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	2,311	2,314
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	5	13
Available-for-sale investments with unrealized losses, Fair Value, Total	2,311	3,677
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	5	19
Corporate bonds and direct obligations of government agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	15,429	28,809
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	21	195
Available-for-sale investments with unrealized losses, 12 months or longer, Fair Value	13,215	1,367
Available-for-sale investments with unrealized losses, 12 months or longer, Unrealized Loss	70	44
Available-for-sale investments with unrealized losses, Fair Value, Total	28,644	30,176
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	91	239
U.S. Treasury obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments with unrealized losses, Less than 12 months, Fair Value	5,495	12,734
Available-for-sale investments with unrealized losses, Less than 12 months, Unrealized Loss	1	6
Available-for-sale investments with unrealized losses, Fair Value, Total	5,495	12,734
Available-for-sale investments with unrealized losses, Unrealized Loss, Total	$ 1	$ 6