Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Level 1 [Member]
Assets
Total assets		$ 95,896	$ 14,950
Liabilities
Convertible notes payable		0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets		95,896	14,950
Level 1 [Member] | Government-backed securities [Member]
Assets
Total assets		0	0
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total assets	[1]	0	0
Level 1 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[2]	0	0
Level 2 [Member]
Assets
Total assets		122,102	186,137
Liabilities
Convertible notes payable		141,989	0
Level 2 [Member] | Money market funds [Member]
Assets
Total assets		0	0
Level 2 [Member] | Government-backed securities [Member]
Assets
Total assets		19,000	20,000
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	[1]	23,632	28,924
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[2]	79,470	137,213
Level 3 [Member]
Assets
Total assets		0	0
Liabilities
Convertible notes payable		0	0
Level 3 [Member] | Money market funds [Member]
Assets
Total assets		0	0
Level 3 [Member] | Government-backed securities [Member]
Assets
Total assets		0	0
Level 3 [Member] | Asset-backed securities [Member]
Assets
Total assets	[1]	0	0
Level 3 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[2]	$ 0	$ 0

[1] Includes $8,185 classified as cash and cash equivalents as of December 31, 2015 (see Note 3).

[2] Includes $11,976 and $20,404 classified as cash and cash equivalents as of December 31, 2016 and 2015, respectively (see Note 3).