Fair Value - Schedule of Financial Assets Measured at Fair Value (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Assets:
Total assets measured and recorded at fair value	$ 179,727
Fair Value	42,057
Cash
Assets:
Cash and cash equivalents	129,513
Money Market Fund
Assets:
Cash and cash equivalents	8,157
Corporate Bonds and Commercial Paper
Assets:
Fair Value	37,090
U.S. Government Securities
Assets:
Fair Value	4,967
Level 1
Assets:
Total assets measured and recorded at fair value	137,670
Level 1 | Cash
Assets:
Cash and cash equivalents	129,513
Level 1 | Money Market Fund
Assets:
Cash and cash equivalents	8,157
Level 2
Assets:
Total assets measured and recorded at fair value	42,057
Level 2 | Corporate Bonds and Commercial Paper
Assets:
Fair Value	37,090
Level 2 | U.S. Government Securities
Assets:
Fair Value	$ 4,967