Amortized Costs and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	$ 28,869	$ 130,936
Available-for-sale securities, Gross Unrealized Gains	844	662
Available-for-sale securities, Gross Unrealized Losses	(121)	(654)
Available-for-sale securities, Fair Value	29,592	130,944
Available-for-sale securities, Less than 12 months, Fair Value	29,592	130,944
Available-for-sale securities, Less than 12 months, Unrealized Losses	(121)	(654)
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	29,592	130,944
Total, Unrealized Losses	(121)	(654)
Certificates of deposit (non-U.S.)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	12,809	87,372
Available-for-sale securities, Fair Value	12,809	87,372
Available-for-sale securities, Less than 12 months, Fair Value	12,809	87,372
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	12,809	87,372
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	11,003	34,175
Available-for-sale securities, Gross Unrealized Gains	86	35
Available-for-sale securities, Gross Unrealized Losses	(24)	(77)
Available-for-sale securities, Fair Value	11,065	34,133
Available-for-sale securities, Less than 12 months, Fair Value	11,065	34,133
Available-for-sale securities, Less than 12 months, Unrealized Losses	(24)	(77)
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	11,065	34,133
Total, Unrealized Losses	(24)	(77)
Short-term bond fund
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs		5,046
Available-for-sale securities, Gross Unrealized Gains		69
Available-for-sale securities, Gross Unrealized Losses		(69)
Available-for-sale securities, Fair Value		5,046
Available-for-sale securities, Less than 12 months, Fair Value		5,046
Available-for-sale securities, Less than 12 months, Unrealized Losses		(69)
Available-for-sale securities, 12 months or more, Fair Value		0
Available-for-sale securities, 12 months or more, Unrealized Losses		0
Total, Fair Value		5,046
Total, Unrealized Losses		(69)
Corporate obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	13	3
Available-for-sale securities, Fair Value	13	3
Available-for-sale securities, Less than 12 months, Fair Value	13	3
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	13	3
Money markets
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	38	54
Available-for-sale securities, Fair Value	38	54
Available-for-sale securities, Less than 12 months, Fair Value	38	54
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	38	54
Mutual funds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	65	94
Available-for-sale securities, Gross Unrealized Gains	14	28
Available-for-sale securities, Gross Unrealized Losses		(21)
Available-for-sale securities, Fair Value	79	101
Available-for-sale securities, Less than 12 months, Fair Value	79	101
Available-for-sale securities, Less than 12 months, Unrealized Losses		(21)
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	79	101
Total, Unrealized Losses		(21)
Other investments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Costs	4,941	4,192
Available-for-sale securities, Gross Unrealized Gains	744	530
Available-for-sale securities, Gross Unrealized Losses	(97)	(487)
Available-for-sale securities, Fair Value	5,588	4,235
Available-for-sale securities, Less than 12 months, Fair Value	5,588	4,235
Available-for-sale securities, Less than 12 months, Unrealized Losses	(97)	(487)
Available-for-sale securities, 12 months or more, Fair Value	0	0
Available-for-sale securities, 12 months or more, Unrealized Losses	0	0
Total, Fair Value	5,588	4,235
Total, Unrealized Losses	$ (97)	$ (487)

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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