Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Level 1 [Member]
Assets
Total assets		$ 36,762	$ 95,896
Liabilities
Convertible notes payable		0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	[1]	36,762	95,896
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		110,059	122,102
Liabilities
Convertible notes payable		152,396	141,989
Level 2 [Member] | Money market funds [Member]
Assets
Total assets	[1]	0	0
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	[2]	29,750	42,632
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	80,309	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] | 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 December 31, 2017 and 2016, respectively (see Note 3).

[2] Includes $16,007 and $19,000 classified as cash and cash equivalents as of December 31, 2017 and 2016, respectively (see Note 3).

[3] Includes $43,056 and $11,976 classified as cash and cash equivalents as of December 31, 2017 and 2016, respectively (see Note 3).