Fair Value Measurements - Financial Assets and Liabilities Measured and Reported at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Assets:
Foreign currency forward exchange contracts	$ 9,776	$ 7,804	$ 4,084
Available-for-sale security	6,657	8,991	7,354
Total assets	16,433	16,795	11,438
Liabilities:
Foreign currency forward exchange contracts	9,792	19,320	29,877
Level 1
Assets:
Foreign currency forward exchange contracts	0	0	0
Available-for-sale security	6,657	8,991	7,354
Total assets	6,657	8,991	7,354
Liabilities:
Foreign currency forward exchange contracts	0	0	0
Level 2
Assets:
Foreign currency forward exchange contracts	9,776	7,804	4,084
Available-for-sale security	0	0	0
Total assets	9,776	7,804	4,084
Liabilities:
Foreign currency forward exchange contracts	9,792	19,320	29,877
Level 3
Assets:
Foreign currency forward exchange contracts	0	0	0
Available-for-sale security	0	0	0
Total assets	0	0	0
Liabilities:
Foreign currency forward exchange contracts	$ 0	$ 0	$ 0