Investments in Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 257,500	$ 255,046
Gross Unrealized Gains	4,125	13,678
Gross Unrealized Losses	(7,526)	(4,281)
Fair Value	254,099us-gaap_AvailableForSaleSecurities	264,443us-gaap_AvailableForSaleSecurities
Total Investments	248,954	248,414
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		65,002
Available-for-sale Debt Securities Gross Unrealized Gain		11
Available-for-sale Debt Securities, Gross Unrealized Loss		(75)
Fair Value		64,938
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	253,590	184,773
Available-for-sale Debt Securities Gross Unrealized Gain		7
Available-for-sale Debt Securities, Gross Unrealized Loss	(4,636)	(1,304)
Fair Value	248,954	183,476
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	3,910	5,271
Available-for-sale Equity Securities, Gross Unrealized Gain	4,125	13,660
Available-for-sale Equity Securities, Gross Unrealized Loss	(2,890)	(2,902)
Fair Value	5,145	16,029
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency securities		64,938
Corporate notes and bonds	248,954	183,476
Total Investments	248,954	248,414
Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity securities	$ 5,145	$ 16,029

Amount of unrealized gain before deducting unrealized loss on investments in debt securities classified as available-for-sale securities.

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