Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Amortized cost	$ 124,560	$ 147,370
Gross Unrealized Gains
Gross Unrealized Gains	15	13
Gross Unrealized Losses
Gross Unrealized Losses	(49)	(68)
Fair Value
Fair Value	124,526	147,315
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	500	600
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	35,515	52,754
Maturing after one year through three years	153	296
Amortized cost	35,668	53,050
Gross Unrealized Gains
Maturing in one year or less	2	5
Maturing after one year through three years	6	8
Gross Unrealized Gains	8	13
Gross Unrealized Losses
Maturing in one year or less	(9)	(12)
Gross Unrealized Losses	(9)	(12)
Fair Value
Maturing in one year or less	35,508	52,747
Maturing after one year through three years	159	304
Fair Value	35,667	53,051
Corporate debt securities
Amortized Cost
Maturing in one year or less	83,864	94,320
Maturing after one year through three years	5,028
Amortized cost	88,892	94,320
Gross Unrealized Gains
Maturing in one year or less	4
Maturing after one year through three years	3
Gross Unrealized Gains	7
Gross Unrealized Losses
Maturing in one year or less	(38)	(56)
Maturing after one year through three years	(2)
Gross Unrealized Losses	(40)	(56)
Fair Value
Maturing in one year or less	83,830	94,264
Maturing after one year through three years	5,029
Fair Value	$ 88,859	$ 94,264