Marketable Securities - (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale of Available-for-sale Securities, Debt	$ 475
Available-for-sale Securities, Failed Auction, Value	0	277
Available-for-sale Securities, Amortized Cost Basis	114,820	86,732
Available-for-sale Securities, Fair Value Disclosure	114,857	86,582
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	102,567	81,495
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	102,595	81,533
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	12,253	4,762
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	12,262	4,772
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	0	0
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	0	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	0	475
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	0	277
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	114,820	86,257
Available-for-sale Securities, Gross Unrealized Gains	39	50
Available-for-sale Securities, Gross Unrealized Losses	2	2
Available-for-sale Securities, Fair Value Disclosure	114,857	86,305
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,965	16,448
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	2	2
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]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	0	475
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	198
Available-for-sale Securities, Fair Value Disclosure	0	277
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	0	277
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	198
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	114,820	86,732
Available-for-sale Securities, Gross Unrealized Gains	39	50
Available-for-sale Securities, Gross Unrealized Losses	2	200
Available-for-sale Securities, Fair Value Disclosure	114,857	86,582
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,965	16,448
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	2	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	277
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	$ 0	$ 198