Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
2021 Convertible Notes [Member]
Additional Information in Fair Value Disclosure [Abstract]
Fair value convertible note	$ 46,900
Recurring [Member]
Assets [Abstract]
Cash and cash equivalents	40,890
Fair value of financial instruments		$ 132,329
Recurring [Member] | Short-term Investments [Member]
Assets [Abstract]
Fair value of financial instruments	233,840
Recurring [Member] | Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	30,905	30,552
Recurring [Member] | United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	9,985
Investments		101,777
Recurring [Member] | United States Treasury Securities [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	121,946
Recurring [Member] | Government Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	35,754
Recurring [Member] | Corporate Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	26,250
Recurring [Member] | Municipal Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	9,000
Recurring [Member] | Investments [Member]
Assets [Abstract]
Investments		101,777
Recurring [Member] | Investments [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	192,950
Recurring [Member] | Level 1 [Member]
Assets [Abstract]
Cash and cash equivalents	40,890
Fair value of financial instruments		132,329
Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Assets [Abstract]
Fair value of financial instruments	162,836
Recurring [Member] | Level 1 [Member] | Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	30,905	30,552
Recurring [Member] | Level 1 [Member] | United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	9,985
Investments		101,777
Recurring [Member] | Level 1 [Member] | United States Treasury Securities [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	121,946
Recurring [Member] | Level 1 [Member] | Government Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 1 [Member] | Corporate Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 1 [Member] | Investments [Member]
Assets [Abstract]
Investments		101,777
Recurring [Member] | Level 1 [Member] | Investments [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	121,946
Recurring [Member] | Level 2 [Member]
Assets [Abstract]
Cash and cash equivalents	0
Fair value of financial instruments		0
Recurring [Member] | Level 2 [Member] | Short-term Investments [Member]
Assets [Abstract]
Fair value of financial instruments	71,004
Recurring [Member] | Level 2 [Member] | Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Recurring [Member] | Level 2 [Member] | United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	0
Investments		0
Recurring [Member] | Level 2 [Member] | United States Treasury Securities [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 2 [Member] | Government Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	35,754
Recurring [Member] | Level 2 [Member] | Corporate Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	26,250
Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	9,000
Recurring [Member] | Level 2 [Member] | Investments [Member]
Assets [Abstract]
Investments		0
Recurring [Member] | Level 2 [Member] | Investments [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	71,004
Recurring [Member] | Level 3 [Member]
Assets [Abstract]
Cash and cash equivalents	0
Fair value of financial instruments		0
Recurring [Member] | Level 3 [Member] | Short-term Investments [Member]
Assets [Abstract]
Fair value of financial instruments	0
Recurring [Member] | Level 3 [Member] | Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Recurring [Member] | Level 3 [Member] | United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	0
Investments		0
Recurring [Member] | Level 3 [Member] | United States Treasury Securities [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 3 [Member] | Government Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 3 [Member] | Corporate Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	0
Recurring [Member] | Level 3 [Member] | Investments [Member]
Assets [Abstract]
Investments		$ 0
Recurring [Member] | Level 3 [Member] | Investments [Member] | Short-term Investments [Member]
Assets [Abstract]
Investments	$ 0