Note D - Marketable Securities and Time Deposits - Marketable Securities and Time Deposits (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Marketable securities and time deposits, amortized cost	$ 109,299	$ 108,575
Marketable securities and time deposits, unrealized gains (losses), net	(1,995)	(1,209)
Marketable securities and time deposits, estimated fair value	107,304	107,366
Short-term Deposits [Member]
Marketable securities and time deposits, amortized cost	5,342	5,481
Marketable securities and time deposits, unrealized gains (losses), net
Marketable securities and time deposits, estimated fair value	5,342	5,481
Long-Term Deposit [Member]
Marketable securities and time deposits, amortized cost	5,071	5,013
Marketable securities and time deposits, unrealized gains (losses), net
Marketable securities and time deposits, estimated fair value	5,071	5,013
US Government-sponsored Enterprises Debt Securities [Member]
Marketable securities and time deposits, amortized cost	22,075	22,359
Marketable securities and time deposits, unrealized gains (losses), net	(489)	(315)
Marketable securities and time deposits, estimated fair value	21,586	22,044
Corporate Debt Securities [Member]
Marketable securities and time deposits, amortized cost	76,811	75,722
Marketable securities and time deposits, unrealized gains (losses), net	(1,506)	(894)
Marketable securities and time deposits, estimated fair value	$ 75,305	$ 74,828