Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Foreign exchange contracts	$ 37,800	$ 39,000
Total cash equivalents	592,148	517,723
Cash collateral for foreign exchange contracts	11,000	12,500
Liabilities:
Foreign exchange contracts	23,900	26,900
Total liabilities	23,903	26,888
Cash collateral obligation for foreign exchange contracts	10,600	10,900
Foreign exchange contracts
Assets:
Foreign exchange contracts	37,849	39,045
Cash collateral for foreign exchange contracts	11,046	12,540
Liabilities:
Foreign exchange contracts	23,903	26,888
Cash collateral obligation for foreign exchange contracts	10,586	10,882
Repurchase agreements
Assets:
Total cash equivalents	498,216	420,838
Money market
Assets:
Total cash equivalents	50,462	50,423
Certificates of deposit
Assets:
Total cash equivalents	5,621	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	592,148	517,713
Liabilities:
Total liabilities	23,903	26,821
Level 2 | Foreign exchange contracts
Assets:
Foreign exchange contracts	37,849	39,035
Liabilities:
Foreign exchange contracts	23,903	26,821
Level 2 | Repurchase agreements
Assets:
Total cash equivalents	498,216	420,838
Level 2 | Money market
Assets:
Total cash equivalents	50,462	50,423
Level 2 | Certificates of deposit
Assets:
Total cash equivalents	$ 5,621	$ 7,417