Investments (Details 2) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	$ 45,636	$ 29,592
Less than 12 months, Unrealized Losses	(571)	(121)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	45,636	29,592
Total, Unrealized Losses	(571)	(121)
Certificates of deposit (non-U.S.)
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	39,640	12,809
Less than 12 months, Unrealized Losses	0	0
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	39,640	12,809
Total, Unrealized Losses	0	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value		11,065
Less than 12 months, Unrealized Losses		(24)
12 months or more, Fair Value		0
12 months or more, Unrealized Losses		0
Total, Fair Value		11,065
Total, Unrealized Losses		(24)
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	14	13
Less than 12 months, Unrealized Losses	0	0
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	14	13
Total, Unrealized Losses	0	0
Money markets
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	37	38
Less than 12 months, Unrealized Losses	0	0
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	37	38
Total, Unrealized Losses	0	0
Mutual funds
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	86	79
Less than 12 months, Unrealized Losses	(1)	0
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	86	79
Total, Unrealized Losses	(1)	0
Other investments
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	5,859	5,588
Less than 12 months, Unrealized Losses	(570)	(97)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	5,859	5,588
Total, Unrealized Losses	$ (570)	$ (97)