Marketable Securities - (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Failed Auction, Value	$ 302	$ 346
Unrealized Gain (Loss) on Investments	173
Available-for-sale Securities, Amortized Cost Basis	91,174	65,063
Available-for-sale Securities, Fair Value Disclosure	91,033	64,963
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	78,692	57,367
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	78,723	57,390
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	9,731	7,016
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	9,732	7,022
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	738	205
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	738	205
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	2,013	475
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,840	346
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	90,199	64,088
Available-for-sale Securities, Gross Unrealized Gains	40	36
Available-for-sale Securities, Gross Unrealized Losses	(8)	(7)
Available-for-sale Securities, Fair Value Disclosure	90,231	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	11,175	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	500	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	500	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	(173)	(129)
Available-for-sale Securities, Fair Value Disclosure	302	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	302	346
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(173)	(129)
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	91,174	65,063
Available-for-sale Securities, Gross Unrealized Gains	40	36
Available-for-sale Securities, Gross Unrealized Losses	(181)	(136)
Available-for-sale Securities, Fair Value Disclosure	91,033	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	11,175	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	302	346
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	$ (173)	$ (129)