Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 70,444	$ 70,408	$ 83,084
Long-term marketable securities	70,114	79,250	46,251
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	70,444	70,408	83,084
Long-term marketable securities	70,114	79,250	46,251
Total marketable securities	140,558	149,658	129,335
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	35,123	40,240	48,240
Long-term marketable securities	61,247	65,109	39,415
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	24,988	24,980	19,986
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	2,052	2,057	9,825
Long-term marketable securities	8,867	8,909	3,625
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	8,281	3,131	5,033
Long-term marketable securities	$ 0	$ 5,232	$ 3,211