Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Level 1 [Member]
Assets
Total cash equivalents and marketable securities		$ 14,950	$ 20,354
Level 1 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities		14,950	20,354
Level 1 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities		0	0
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	[1]	0	0
Level 1 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	[2]	0	0
Level 2 [Member]
Assets
Total cash equivalents and marketable securities		186,137	143,221
Level 2 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities		0	0
Level 2 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities		20,000	7,500
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	[1]	28,924	46,624
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	[2]	137,213	89,097
Level 3 [Member]
Assets
Total cash equivalents and marketable securities		0	0
Level 3 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities		0	0
Level 3 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities		0	0
Level 3 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	[1]	0	0
Level 3 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	[2]	$ 0	$ 0

[1] Includes $8,185 classified as cash and cash equivalents (See Note 3).

[2] Includes $20,404 classified as cash and cash equivalents (See Note 3).