Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 87,929	$ 116,924
Available-for-sale Securities, Fair Value Disclosure		87,872	116,924
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		82,498	113,542
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value		82,445	113,549
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis		5,431	3,382
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value		5,427	3,375
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis		0	0
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value		0	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis		0	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value		0	0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		87,088	116,086
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		6	18
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(65)	(15)
Available-for-sale Securities, Fair Value Disclosure		87,029	116,089
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 64,918	52,638
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(65)	(15)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	305
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(1)
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	842	838
Available-for-sale Securities, Gross Unrealized Gain	1	0
Available-for-sale Securities, Gross Unrealized Loss	0	(3)
Available-for-sale Securities, Fair Value Disclosure	843	835
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		87,930	116,924
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		7	18
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(65)	(18)
Available-for-sale Securities, Fair Value Disclosure		87,872	$ 116,924
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	64,918	53,473
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(65)	(18)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	305
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0	$ (1)