Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 21,271		$ 56,088		$ 43,260
Long-term marketable securities	55,225		55,342		45,800
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	21,271		56,088		43,260
Long-term marketable securities	55,225		55,342		45,800
Total marketable securities	76,496	[1]	111,430	[1]	89,060	[1]
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	613		7,375		20,107
Long-term marketable securities	55,225		55,342		40,041
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	14,992		34,966		14,967
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,666		8,738		3,065
Long-term marketable securities	0		0		5,759
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 0		$ 5,009		$ 5,121

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