Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 56,088	$ 70,408
Long-term marketable securities	55,342	79,250
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	56,088	70,408
Long-term marketable securities	55,342	79,250
Total marketable securities	111,430	149,658
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	7,375	40,240
Long-term marketable securities	55,342	65,109
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	34,966	24,980
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	8,738	2,057
Long-term marketable securities	0	8,909
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,009	3,131
Long-term marketable securities	$ 0	$ 5,232