Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Level 1 [Member]
Assets
Total assets		$ 36,047	$ 95,896
Liabilities
Convertible notes payable		0	0
Level 1 [Member] | Money Market Funds [Member]
Assets
Total assets	[1]	36,047	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		122,378	122,102
Liabilities
Convertible notes payable		145,805	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]	28,451	42,632
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	93,927	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 September 30, 2017 and December 31, 2016, respectively (see Note 3).

[2] Includes $14,000 and $19,000 classified as cash and cash equivalents as of September 30, 2017 and December 31, 2016, respectively (see Note 3).

[3] Includes $40,057 and $11,976 classified as cash and cash equivalents as of September 30, 2017 and December 31, 2016, respectively (see Note 3).