Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 70,408	$ 109,447
Long-term marketable securities	79,250	34,259
Total marketable securities	149,658	143,706
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	70,408	95,036
Long-term marketable securities	79,250	34,259
Total marketable securities	149,658	129,295
Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	14,411
Long-term marketable securities	0	0
Total marketable securities	0	14,411
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	40,240	40,047
Long-term marketable securities	65,109	30,618
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	40,240	40,047
Long-term marketable securities	65,109	30,618
U.S. Government and agency obligations [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0
Long-term marketable securities	0	0
Commercial paper [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	24,980	33,971
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	24,980	33,971
Commercial paper [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0
Municipal bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	2,057	10,896
Long-term marketable securities	8,909	3,641
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	2,057	10,896
Long-term marketable securities	8,909	3,641
Municipal bonds [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	0
Long-term marketable securities	0	0
Corporate bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	3,131	10,122
Long-term marketable securities	5,232
Corporate bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	3,131	10,122
Long-term marketable securities	5,232
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
Equity securities - mutual funds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities		0
Equity securities - mutual funds [Member] | Trading [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities		$ 14,411