Note E - Marketable Securities and Time Deposits - Marketable Securities and Time Deposits (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Marketable securities and time deposits, amortized cost	$ 107,844	$ 112,885
Marketable securities and time deposits, unrealized gains (losses), net	(242)	(1,635)
Marketable securities and time deposits, estimated fair value	107,602	111,251
Short-term Deposits [Member]
Marketable securities and time deposits, amortized cost	5,467	8,349
Marketable securities and time deposits, unrealized gains (losses), net
Marketable securities and time deposits, estimated fair value	5,467	8,349
Long-Term Deposit [Member]
Marketable securities and time deposits, amortized cost	5,188	5,130
Marketable securities and time deposits, unrealized gains (losses), net
Marketable securities and time deposits, estimated fair value	5,188	5,130
US Government-sponsored Enterprises Debt Securities [Member]
Marketable securities and time deposits, amortized cost	17,158	21,550
Marketable securities and time deposits, unrealized gains (losses), net	(41)	(253)
Marketable securities and time deposits, estimated fair value	17,117	21,297
Corporate Debt Securities [Member]
Marketable securities and time deposits, amortized cost	80,031	77,857
Marketable securities and time deposits, unrealized gains (losses), net	(201)	(1,382)
Marketable securities and time deposits, estimated fair value	$ 79,830	$ 76,475