Investments in Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 256,143	$ 255,046
Gross Unrealized Gains	12,207	13,678
Gross Unrealized Losses	(4,853)	(4,281)
Fair Value	263,497	264,443
Short term investments	248,572	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	250,723	184,773
Available-for-sale Debt Securities Gross Unrealized Gain	4	7
Available-for-sale Debt Securities, Gross Unrealized Loss	(2,155)	(1,304)
Fair Value	248,572	183,476
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	5,420	5,271
Available-for-sale Equity Securities, Gross Unrealized Gain	12,203	13,660
Available-for-sale Equity Securities, Gross Unrealized Loss	(2,698)	(2,902)
Fair Value	14,925	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	248,572	183,476
Equity securities	0	0
Short term investments	248,572	248,414
Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency securities		0
Corporate notes and bonds	0	0
Equity securities	$ 14,925	$ 16,029