Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 47,185	$ 70,408	$ 77,389
Long-term marketable securities	37,802	79,250	59,509
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	47,185	70,408	77,389
Long-term marketable securities	37,802	79,250	59,509
Total marketable securities	84,987	149,658	136,898
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	15,062	40,240	40,218
Long-term marketable securities	36,103	65,109	45,268
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	19,976	24,980	29,978
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	7,082	2,057	4,036
Long-term marketable securities	1,699	8,909	8,952
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,065	3,131	3,157
Long-term marketable securities	$ 0	$ 5,232	$ 5,289