Investments - Gross Unrealized Losses and Fair Value for Investments in Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	$ 50,571
Less than 12 months, Unrealized Loss	(35)
Corporate Notes and Obligations
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	16,647
Less than 12 months, Unrealized Loss	(21)
U.S. Government Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	33,924
Less than 12 months, Unrealized Loss	$ (14)