Fair Value of Financial Instruments (Details) - Recurring [Member] $ in Thousands	Jun. 30, 2018 USD ($)
Assets
Fair value of financial instruments	$ 161,759
Money Market Mutual Funds [Member]
Assets
Cash and cash equivalents	42,841
Government Agency Securities [Member]
Assets
Cash and cash equivalents	4,978
United States Treasury Securities [Member]
Assets
Cash and cash equivalents	113,940
Investments [Member]
Assets
Cash and cash equivalents	118,918
Level 1 [Member]
Assets
Fair value of financial instruments	156,781
Level 1 [Member] | Money Market Mutual Funds [Member]
Assets
Cash and cash equivalents	42,841
Level 1 [Member] | Government Agency Securities [Member]
Assets
Cash and cash equivalents	0
Level 1 [Member] | United States Treasury Securities [Member]
Assets
Cash and cash equivalents	113,940
Level 1 [Member] | Investments [Member]
Assets
Cash and cash equivalents	113,940
Level 2 [Member]
Assets
Fair value of financial instruments	4,978
Level 2 [Member] | Money Market Mutual Funds [Member]
Assets
Cash and cash equivalents	0
Level 2 [Member] | Government Agency Securities [Member]
Assets
Cash and cash equivalents	4,978
Level 2 [Member] | United States Treasury Securities [Member]
Assets
Cash and cash equivalents	0
Level 2 [Member] | Investments [Member]
Assets
Cash and cash equivalents	4,978
Level 3 [Member]
Assets
Fair value of financial instruments	0
Level 3 [Member] | Money Market Mutual Funds [Member]
Assets
Cash and cash equivalents	0
Level 3 [Member] | Government Agency Securities [Member]
Assets
Cash and cash equivalents	0
Level 3 [Member] | United States Treasury Securities [Member]
Assets
Cash and cash equivalents	0
Level 3 [Member] | Investments [Member]
Assets
Cash and cash equivalents	$ 0