Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Level 1 [Member]
Assets
Total assets		$ 49,113	$ 39,168
Liabilities
Convertible notes payable		0	0
Level 1 [Member] | Money Market Funds [Member]
Assets
Total assets	[1]	49,113	39,168
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		41,712	46,216
Liabilities
Convertible notes payable		117,267	197,935
Level 2 [Member] | Money Market Funds [Member]
Assets
Total assets	[1]	0	0
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	[2]	15,000	19,997
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	[3]	26,712	26,219
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, 2019 and December 31, 2018, respectively, on the consolidated balance sheets.

[2] Includes $15,000 classified as cash and cash equivalents as of both March 31, 2019 and December 31, 2018 on the consolidated balance sheets.

[3] Includes $24,228 and $9,236 classified as cash and cash equivalents as of March 31, 2019 and December 31, 2018, respectively, on the consolidated balance sheet.