Marketable Securities, Available-for-sale (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Marketable Securities, Available-for-sale
Gain from marketable securities, net	$ 1,849,000	$ 2,214,000	$ 3,882,000
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	313,391,000	385,082,000
GROSS UNREALIZED GAINS	821,000	1,278,000
GROSS UNREALIZED LOSSES	(1,057,000)	(735,000)
ESTIMATED FAIR VALUE	313,155,000	385,625,000
Total debt securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	305,152,000	348,549,000
GROSS UNREALIZED GAINS	821,000	1,278,000
GROSS UNREALIZED LOSSES	(1,057,000)	(728,000)
ESTIMATED FAIR VALUE	304,916,000	349,099,000
U.S. Treasury securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	76,355,000	3,098,000
GROSS UNREALIZED GAINS	149,000	1,000
GROSS UNREALIZED LOSSES	0	0
ESTIMATED FAIR VALUE	76,504,000	3,099,000
Obligations of U.S. government agencies
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	17,074,000	133,029,000
GROSS UNREALIZED GAINS	2,000	112,000
GROSS UNREALIZED LOSSES	(8,000)	(31,000)
ESTIMATED FAIR VALUE	17,068,000	133,110,000
Municipal debt securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	33,492,000	21,745,000
GROSS UNREALIZED GAINS	492,000	896,000
GROSS UNREALIZED LOSSES	(1,008,000)	(458,000)
ESTIMATED FAIR VALUE	32,976,000	22,183,000
Corporate debt securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	157,798,000	163,352,000
GROSS UNREALIZED GAINS	102,000	116,000
GROSS UNREALIZED LOSSES	(21,000)	(75,000)
ESTIMATED FAIR VALUE	157,879,000	163,393,000
Asset-backed securities
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	20,433,000	27,325,000
GROSS UNREALIZED GAINS	76,000	153,000
GROSS UNREALIZED LOSSES	(20,000)	(164,000)
ESTIMATED FAIR VALUE	20,489,000	27,314,000
Time deposits
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	1,606,000
GROSS UNREALIZED GAINS	0
GROSS UNREALIZED LOSSES	0
ESTIMATED FAIR VALUE	1,606,000	0
Short-term pooled investments
Schedule of fair values of available-for-sale marketable securities
AMORTIZED COST	6,633,000	36,533,000
GROSS UNREALIZED GAINS	0	0
GROSS UNREALIZED LOSSES	0	(7,000)
ESTIMATED FAIR VALUE	$ 6,633,000	$ 36,526,000