Fair Value - Schedule of Financial Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Restricted cash	$ 26,350	$ 0
Fair Value	27,401	42,146
Total assets measured and recorded at fair value	290,251	197,710
Level 1
Assets:
Restricted cash	26,350
Total assets measured and recorded at fair value	262,850	155,564
Level 2
Assets:
Total assets measured and recorded at fair value	27,401	42,146
Cash
Assets:
Cash and cash equivalents	213,069	147,221
Cash | Level 1
Assets:
Cash and cash equivalents	213,069	147,221
Money Market Fund
Assets:
Cash and cash equivalents	23,431	8,343
Money Market Fund | Level 1
Assets:
Cash and cash equivalents	23,431	8,343
Corporate Bonds and Commercial Paper
Assets:
Fair Value	22,437	37,151
Corporate Bonds and Commercial Paper | Level 2
Assets:
Fair Value	22,437	37,151
U.S. Government Securities
Assets:
Fair Value	4,964	4,995
U.S. Government Securities | Level 2
Assets:
Fair Value	$ 4,964	$ 4,995