Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 78,255	$ 109,447	$ 73,566
Long-term marketable securities	49,580	34,259	68,291
Total marketable securities	127,835	143,706	141,857
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	49,400	40,047	30,388
Long-term marketable securities	40,144	30,618	60,430
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	49,400	40,047	30,388
Long-term marketable securities	40,144	30,618	60,430
U.S. Government and agency obligations [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0	0
Long-term marketable securities	0	0	0
Commercial paper [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	19,986	33,971	24,969
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	19,986	33,971	24,969
Commercial paper [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0	0
Municipal bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,685	10,896	13,020
Long-term marketable securities	4,091	3,641	7,861
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,685	10,896	13,020
Long-term marketable securities	4,091	3,641	7,861
Municipal bonds [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0	0
Long-term marketable securities	0	0	0
Corporate bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	3,184	10,122
Long-term marketable securities	5,345
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	3,184	10,122
Long-term marketable securities	5,345
Corporate bonds [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0
Long-term marketable securities	0
Equity securities - mutual funds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities		14,411	5,189
Equity securities - mutual funds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities		0	0
Equity securities - mutual funds [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities		14,411	5,189
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	78,255	95,036	68,377
Long-term marketable securities	49,580	34,259	68,291
Total marketable securities	127,835	129,295	136,668
Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	14,411	5,189
Long-term marketable securities	0	0	0
Total marketable securities	$ 0	$ 14,411	$ 5,189