Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Amortized Cost
Amortized cost	$ 237,855	$ 173,014
Gross Unrealized Gains
Gross Unrealized Gains	120	88
Gross Unrealized Losses
Gross Unrealized Losses	(156)	(79)
Fair Value
Fair Value	237,819	173,023
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,200	1,400
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	43,379	44,580
Maturing after one year through three years	22,921	16,108
Amortized cost	66,300	60,688
Gross Unrealized Gains
Maturing in one year or less	21	30
Maturing after one year through three years	54	49
Gross Unrealized Gains	75	79
Gross Unrealized Losses
Maturing in one year or less	(2)	(6)
Maturing after one year through three years	(45)	(1)
Gross Unrealized Losses	(47)	(7)
Fair Value
Maturing in one year or less	43,398	44,604
Maturing after one year through three years	22,930	16,156
Fair Value	66,328	60,760
Corporate debt securities
Amortized Cost
Maturing in one year or less	135,613	89,636
Maturing after one year through three years	35,942	22,690
Amortized cost	171,555	112,326
Gross Unrealized Gains
Maturing in one year or less	24	6
Maturing after one year through three years	21	3
Gross Unrealized Gains	45	9
Gross Unrealized Losses
Maturing in one year or less	(89)	(37)
Maturing after one year through three years	(20)	(35)
Gross Unrealized Losses	(109)	(72)
Fair Value
Maturing in one year or less	135,548	89,605
Maturing after one year through three years	35,943	22,658
Fair Value	$ 171,491	$ 112,263