Summary of Significant Accounting Policies (Details 4) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 item	Dec. 31, 2014 item
Unrealized loss position for less than one year
Number of marketable securities in an unrealized loss position for less than one year	31	44
Number of marketable securities in an unrealized loss position for greater than one year	0	1
Corporate debt securities | Due in one year or less
Unrealized loss position for less than one year
Aggregate Fair Value	36,740	63,221
Unrealized Losses	(8)	(24)
Foreign government bonds | Due in one year or less
Unrealized loss position for less than one year
Aggregate Fair Value	10,714	12,773
Unrealized Losses	(1)	(1)
Asset-backed securities | Due in one year or less
Unrealized loss position for less than one year
Aggregate Fair Value	18,579	25,742
Unrealized Losses	(2)	(9)