Cash, Cash Equivalents and Marketable Securities - Aggregate Fair Value Of Securities (Details) $ in Thousands	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Unrealized loss position for less than one year
Number of marketable securities in an unrealized loss position for less than one year | item	66	44
Number of marketable securities in an unrealized loss position for greater than one year | item	0	1
Corporate debt securities | Due in one year or less
Unrealized loss position for less than one year
Aggregate Fair Value	$ 70,657	$ 63,221
Unrealized Losses	(84)	(24)
Foreign government bonds | Due in one year or less
Unrealized loss position for less than one year
Aggregate Fair Value	33,734
Unrealized Losses	(8)
Asset-backed securities
Unrealized loss position for less than one year
Aggregate Fair Value	61,337	25,742
Unrealized Losses	$ (30)	(9)
Asset-backed securities | Due in one year or less
Unrealized loss position for less than one year
Aggregate Fair Value		12,773
Unrealized Losses		$ (1)