Fair Value Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 82,664	$ 61,868
Foreign exchange contracts		6
Government bonds
Assets:
Marketable securities	495	497
Certificates of deposits
Assets:
Marketable securities	747
Corporate bonds
Assets:
Marketable securities	81,422	61,371
Level II
Assets:
Foreign exchange contracts		6
Level II | Government bonds
Assets:
Marketable securities	495	497
Level II | Certificates of deposits
Assets:
Marketable securities	747
Level II | Corporate bonds
Assets:
Marketable securities	$ 81,422	$ 61,371