Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 22,892		$ 56,088		$ 47,185
Long-term marketable securities	64,014		55,342		37,802
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	22,892		56,088		47,185
Long-term marketable securities	64,014		55,342		37,802
Total marketable securities	86,906	[1]	111,430	[1]	84,987	[1]
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	1,260		7,375		15,062
Long-term marketable securities	64,014		55,342		36,103
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	19,982		34,966		19,976
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	1,650		8,738		7,082
Long-term marketable securities	0		0		1,699
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 0		$ 5,009		$ 5,065

[1] 1Held-to-maturity marketable securities are periodically assessed for other-than-temporary impairment.