Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Foreign exchange contracts	$ 43,000	$ 39,000
Total cash equivalents	609,048	517,723
Cash collateral for foreign exchange contracts	40,100	12,500
Liabilities:
Foreign exchange contracts	41,000	26,900
Total liabilities	40,969	26,888
Cash collateral obligation for foreign exchange contracts	9,900	10,900
Foreign exchange contracts
Assets:
Foreign exchange contracts	43,000	39,045
Cash collateral for foreign exchange contracts	40,084	12,540
Liabilities:
Foreign exchange contracts	40,969	26,888
Cash collateral obligation for foreign exchange contracts	9,917	10,882
Repurchase agreements
Assets:
Total cash equivalents	500,824	420,838
Money market
Assets:
Total cash equivalents	50,187	50,423
Certificates of deposit
Assets:
Total cash equivalents	15,037	7,417
Level 1
Assets:
Total cash equivalents	0	10
Liabilities:
Total liabilities	0	67
Level 1 | Foreign exchange contracts
Assets:
Foreign exchange contracts	0	10
Liabilities:
Foreign exchange contracts	0	67
Level 2
Assets:
Total cash equivalents	609,048	517,713
Liabilities:
Total liabilities	40,969	26,821
Level 2 | Foreign exchange contracts
Assets:
Foreign exchange contracts	43,000	39,035
Liabilities:
Foreign exchange contracts	40,969	26,821
Level 2 | Repurchase agreements
Assets:
Total cash equivalents	500,824	420,838
Level 2 | Money market
Assets:
Total cash equivalents	50,187	50,423
Level 2 | Certificates of deposit
Assets:
Total cash equivalents	$ 15,037	$ 7,417