Marketable Securities, Available-for-sale (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Marketable Securities, Available-for-sale
Gain from marketable securities, net	$ 404	$ 705
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	264,487		313,391
GROSS UNREALIZED GAINS	1,026		821
GROSS UNREALIZED LOSSES	(824)		(1,057)
ESTIMATED FAIR VALUE	264,689		313,155
Total debt securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	252,042		305,152
GROSS UNREALIZED GAINS	1,026		821
GROSS UNREALIZED LOSSES	(824)		(1,057)
ESTIMATED FAIR VALUE	252,244		304,916
U.S. Treasury securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	70,542		76,355
GROSS UNREALIZED GAINS	198		149
GROSS UNREALIZED LOSSES	0		0
ESTIMATED FAIR VALUE	70,740		76,504
Obligations of U.S. government agencies
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	16,288		17,074
GROSS UNREALIZED GAINS	1		2
GROSS UNREALIZED LOSSES	(11)		(8)
ESTIMATED FAIR VALUE	16,278		17,068
Municipal debt securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	28,249		33,492
GROSS UNREALIZED GAINS	655		492
GROSS UNREALIZED LOSSES	(771)		(1,008)
ESTIMATED FAIR VALUE	28,133		32,976
Corporate debt securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	119,178		157,798
GROSS UNREALIZED GAINS	99		102
GROSS UNREALIZED LOSSES	(20)		(21)
ESTIMATED FAIR VALUE	119,257		157,879
Asset-backed securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	17,785		20,433
GROSS UNREALIZED GAINS	73		76
GROSS UNREALIZED LOSSES	(22)		(20)
ESTIMATED FAIR VALUE	17,836		20,489
Time deposits
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	1,408		1,606
GROSS UNREALIZED GAINS	0		0
GROSS UNREALIZED LOSSES	0		0
ESTIMATED FAIR VALUE	1,408		1,606
Short-term pooled investments
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	11,037		6,633
GROSS UNREALIZED GAINS	0		0
GROSS UNREALIZED LOSSES	0		0
ESTIMATED FAIR VALUE	$ 11,037		$ 6,633