Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	$ 72,336	$ 59,469
Available-for-sale Securities, Failed Auction, Value	336	363
Unrealized Gain (Loss) on Investments	164
Available-for-sale Securities, Fair Value Disclosure	84,649	70,888
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis	11,292	10,587
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	521	314
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	604	605
Available-for-sale Securities	84,753	70,975
Auction Rate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	500	500
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	(164)	(137)
Available-for-sale Securities, Fair Value Disclosure	336	363
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	84,253	70,475
Available-for-sale Securities, Gross Unrealized Gains	70	60
Available-for-sale Securities, Gross Unrealized Losses	(10)	(10)
Available-for-sale Securities, Fair Value Disclosure	84,313	70,525
Fair Value, Disclosure Item Amounts [Domain]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	72,376	59,509
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value	11,317	10,599
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	517	312
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	439	468
Available-for-sale Securities	84,649	70,888
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	84,753	70,975
Available-for-sale Securities, Gross Unrealized Gains	70	60
Available-for-sale Securities, Gross Unrealized Losses	(174)	(147)
Available-for-sale Securities, Fair Value Disclosure	84,649	70,888
US Government and Government Agencies and Authorities [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	14,835	7,291
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(10)	(10)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	0
Auction Rate Securities [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	336	363
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(164)	(137)
Available-for-sale Securities [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	14,835	7,291
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(10)	(10)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	336	363
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	$ (164)	$ (137)