Investments in Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 255,633	$ 240,382
Gross Unrealized Losses	5,548	2,706
Gross Unrealized Losses	(4,723)	(3,436)
Fair Value	256,458	239,652
Total Investments	248,414	234,305
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	135,457	146,993
Gross Unrealized Losses	1	2
Gross Unrealized Losses	(705)	(412)
Fair Value	134,753	146,583
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	114,905	88,118
Gross Unrealized Losses
Gross Unrealized Losses	(1,244)	(396)
Fair Value	113,661	87,722
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	5,271	5,271
Gross Unrealized Losses	5,547	2,704
Gross Unrealized Losses	(2,774)	(2,628)
Fair Value	8,044	5,347
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency securities	134,753	146,583
Corporate notes and bonds	113,661	87,722
Total Investments	248,414	234,305
Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity securities	$ 8,044	$ 5,347