Investments and Fair Value Measurements - Summary of Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Summary of Investments [Abstract]
Cost	$ 120,746
Gross unrealized gains	35
Gross unrealized losses	(16)
Estimated fair value	120,765
Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	112,785	$ 167,348
Gross unrealized gains	27
Gross unrealized losses	(16)	(193)
Estimated fair value	112,796	167,155
Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	7,961
Gross unrealized gains	8
Estimated fair value	7,969
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	54,414	81,770
Gross unrealized gains	4
Gross unrealized losses	(14)	(151)
Estimated fair value	54,404	81,619
Debt Securities Issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	58,371	85,578
Gross unrealized gains	23
Gross unrealized losses	(2)	(42)
Estimated fair value	58,392	$ 85,536
Debt Securities Issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	7,961
Gross unrealized gains	8
Estimated fair value	$ 7,969