Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 77,389	$ 109,447	$ 80,962
Long-term marketable securities	59,509	34,259	49,502
Total marketable securities	136,898	143,706	130,464
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	40,218	40,047	44,407
Long-term marketable securities	45,268	30,618	45,844
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	40,218	40,047	44,407
Long-term marketable securities	45,268	30,618	45,844
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	29,978	33,971	19,971
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	29,978	33,971	19,971
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	4,036	10,896	11,204
Long-term marketable securities	8,952	3,641	3,658
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	4,036	10,896	11,204
Long-term marketable securities	8,952	3,641	3,658
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	3,157	10,122
Long-term marketable securities	5,289
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	3,157	10,122
Long-term marketable securities	5,289
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	5,380
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	5,380
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	77,389	95,036	75,582
Long-term marketable securities	59,509	34,259	49,502
Total marketable securities	136,898	129,295	125,084
Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	14,411	5,380
Long-term marketable securities	0	0	0
Total marketable securities	$ 0	$ 14,411	$ 5,380