Marketable Securities - (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Failed Auction, Value	$ 330	$ 346
Unrealized Gain (Loss) on Investments	145
Available-for-sale Securities, Amortized Cost Basis	81,406	65,063
Available-for-sale Securities, Fair Value Disclosure	81,320	64,963
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	71,837	57,367
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	71,886	57,390
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	6,165	7,016
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	6,171	7,022
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	1,392	205
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	1,395	205
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	2,012	475
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,868	346
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	80,430	64,088
Available-for-sale Securities, Gross Unrealized Gains	67	36
Available-for-sale Securities, Gross Unrealized Losses	(8)	(7)
Available-for-sale Securities, Fair Value Disclosure	80,489	64,117
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	7,559	9,037
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(8)	(7)
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
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	501	500
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value Disclosure	501	500
Auction Rate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	475	475
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	(145)	(129)
Available-for-sale Securities, Fair Value Disclosure	330	346
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	330	346
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(145)	(129)
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	81,406	65,063
Available-for-sale Securities, Gross Unrealized Gains	67	36
Available-for-sale Securities, Gross Unrealized Losses	(153)	(136)
Available-for-sale Securities, Fair Value Disclosure	81,320	64,963
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	7,559	9,037
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(8)	(7)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	330	346
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	$ (145)	$ (129)