Fair Value - Schedule of Financial Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Restricted cash	$ 868	$ 0
Fair Value	1,496	42,146
Total assets measured and recorded at fair value	388,363	197,710
Level 1
Assets:
Restricted cash	868
Total assets measured and recorded at fair value	386,867	155,564
Level 2
Assets:
Total assets measured and recorded at fair value	1,496	42,146
Cash
Assets:
Cash and cash equivalents	336,053	147,221
Cash | Level 1
Assets:
Cash and cash equivalents	336,053	147,221
Money Market Fund
Assets:
Cash and cash equivalents	49,946	8,343
Money Market Fund | Level 1
Assets:
Cash and cash equivalents	49,946	8,343
Corporate Bonds and Commercial Paper
Assets:
Fair Value	1,496	37,151
Corporate Bonds and Commercial Paper | Level 2
Assets:
Fair Value	$ 1,496	37,151
U.S. Government Securities
Assets:
Fair Value		4,995
U.S. Government Securities | Level 2
Assets:
Fair Value		$ 4,995