Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Level 1 [Member]
Assets
Total assets		$ 31,056	$ 36,762
Liabilities
Convertible notes payable		0	0
Level 1 [Member] | Money Market Funds [Member]
Assets
Total assets	[1]	31,056	36,762
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		99,256	110,059
Liabilities
Convertible notes payable		228,621	152,396
Level 2 [Member] | Money Market Funds [Member]
Assets
Total assets	[1]	0	0
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	[2]	28,496	29,750
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	70,760	80,309
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 March 31, 2018 and December 31, 2017, respectively (see Note 3).

[2] Includes $18,750 and $16,007 classified as cash and cash equivalents as of March 31, 2018 and December 31, 2017, respectively (see Note 3).

[3] Includes $50,148 and $43,056 classified as cash and cash equivalents as of March 31, 2018 and December 31, 2017, respectively (see Note 3).