Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 43,260	$ 70,408	$ 78,255
Long-term marketable securities	45,800	79,250	49,580
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	43,260	70,408	78,255
Long-term marketable securities	45,800	79,250	49,580
Total marketable securities	89,060	149,658	127,835
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	20,107	40,240	49,400
Long-term marketable securities	40,041	65,109	40,144
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	14,967	24,980	19,986
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	3,065	2,057	5,685
Long-term marketable securities	5,759	8,909	4,091
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,121	3,131	3,184
Long-term marketable securities	$ 0	$ 5,232	$ 5,345