Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Level 1 [Member]
Assets
Total assets		$ 51,816	$ 95,896
Liabilities
Convertible notes payable		0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	[1]	51,816	95,896
Level 1 [Member] | Government-backed securities [Member]
Assets
Total assets		0	0
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total assets	[2]	0	0
Level 1 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	0	0
Level 2 [Member]
Assets
Total assets		143,822	122,102
Liabilities
Convertible notes payable		148,671	141,989
Level 2 [Member] | Money market funds [Member]
Assets
Total assets	[1]	0	0
Level 2 [Member] | Government-backed securities [Member]
Assets
Total assets		19,000	19,000
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	[2]	48,573	23,632
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	76,249	79,470
Level 3 [Member]
Assets
Total assets		0	0
Liabilities
Convertible notes payable		0	0
Level 3 [Member] | Money market funds [Member]
Assets
Total assets	[1]	0	0
Level 3 [Member] | Government-backed securities [Member]
Assets
Total assets		0	0
Level 3 [Member] | Asset-backed securities [Member]
Assets
Total assets	[2]	0	0
Level 3 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	$ 0	$ 0

[1] Classified as cash and cash equivalents as of March 31, 2017 and December 31, 2016, respectively (see Note 3).

[2] Includes $3,500 classified as cash and cash equivalents as of March 31, 2017 (see Note 3).

[3] Includes $5,991 and $11,976 classified as cash and cash equivalents as of March 31, 2017 and December 31, 2016, respectively (see Note 3).