Marketable Securities - (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 91,230		$ 87,930
Available-for-sale Securities, Fair Value Disclosure	91,244		87,872
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	77,592		82,498
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	77,601		82,445
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	13,638		5,432
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	13,643		5,427
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		0
Available-for-sale 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	90,389		87,088
Available-for-sale Securities, Gross Unrealized Gains	21	$ 6
Available-for-sale Securities, Gross Unrealized Losses	(7)	(65)
Available-for-sale Securities, Fair Value Disclosure	90,403		87,029
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	26,062		64,918
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(7)	(65)
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	841		842
Available-for-sale Securities, Gross Unrealized Gains	0	1
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value Disclosure	841		843
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		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	0
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	91,230		87,930
Available-for-sale Securities, Gross Unrealized Gains	21	7
Available-for-sale Securities, Gross Unrealized Losses	(7)	(65)
Available-for-sale Securities, Fair Value Disclosure	91,244		87,872
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	26,062		64,918
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(7)	(65)
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