Summary of Significant Accounting Policies (Details 4) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 Item	Dec. 31, 2012 Item
U.S. Government-sponsored enterprise obligations
Unrealized loss position for less than one year
Aggregate Fair Value	$ 21,781
Unrealized Losses	(27)
Corporate debt securities
Unrealized loss position for less than one year
Number of marketable securities in an unrealized loss position for less than one year	31	28
Number of marketable securities	0	0
Aggregate Fair Value	80,226	82,529
Unrealized Losses	(72)	(41)
Foreign government bonds
Unrealized loss position for less than one year
Aggregate Fair Value	8,282	8,445
Unrealized Losses	$ (1)	$ (3)