Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Amortized Cost
Amortized cost	$ 252,568	$ 173,014
Gross Unrealized Gains
Gross Unrealized Gains	73	88
Gross Unrealized Losses
Gross Unrealized Losses	(233)	(79)
Fair Value
Fair Value	252,408	173,023
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,600	1,400
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	56,882	44,580
Maturing after one year through three years	12,120	16,108
Amortized cost	69,002	60,688
Gross Unrealized Gains
Maturing in one year or less	22	30
Maturing after one year through three years	30	49
Gross Unrealized Gains	52	79
Gross Unrealized Losses
Maturing in one year or less	(3)	(6)
Maturing after one year through three years	(53)	(1)
Gross Unrealized Losses	(56)	(7)
Fair Value
Maturing in one year or less	56,901	44,604
Maturing after one year through three years	12,097	16,156
Fair Value	68,998	60,760
Corporate debt securities
Amortized Cost
Maturing in one year or less	120,267	89,636
Maturing after one year through three years	63,299	22,690
Amortized cost	183,566	112,326
Gross Unrealized Gains
Maturing in one year or less	12	6
Maturing after one year through three years	9	3
Gross Unrealized Gains	21	9
Gross Unrealized Losses
Maturing in one year or less	(92)	(37)
Maturing after one year through three years	(85)	(35)
Gross Unrealized Losses	(177)	(72)
Fair Value
Maturing in one year or less	120,187	89,605
Maturing after one year through three years	63,223	22,658
Fair Value	$ 183,410	$ 112,263