Marketable Securities - (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 84,781	$ 116,924
Available-for-sale Securities, Fair Value Disclosure	84,715	116,924
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	79,688	113,542
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	79,627	113,549
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	5,093	3,382
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	5,088	3,375
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	83,943	116,086
Available-for-sale Securities, Gross Unrealized Gains	2	18
Available-for-sale Securities, Gross Unrealized Losses	67	15
Available-for-sale Securities, Fair Value Disclosure	83,878	116,089
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	70,042	52,638
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	67	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, Aggregate Losses	0	1
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	838	838
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	1	3
Available-for-sale Securities, Fair Value Disclosure	837	835
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	837	835
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1	3
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	84,781	116,924
Available-for-sale Securities, Gross Unrealized Gains	2	18
Available-for-sale Securities, Gross Unrealized Losses	68	18
Available-for-sale Securities, Fair Value Disclosure	84,715	116,924
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	70,879	53,473
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	68	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, Aggregate Losses	$ 0	$ 1