Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 44,841	$ 56,088	$ 70,444
Long-term marketable securities	57,958	55,342	70,114
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	44,841	56,088	70,444
Long-term marketable securities	57,958	55,342	70,114
Total marketable securities	102,799	111,430	140,558
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	6,169	7,375	35,123
Long-term marketable securities	57,958	55,342	61,247
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	29,976	34,966	24,988
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	8,696	8,738	2,052
Long-term marketable securities	0	0	8,867
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 0	$ 5,009	$ 8,281