Investments - Gross Unrealized Losses and Fair Value for Investments in Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	$ 57,273
Less than 12 months, Unrealized Loss	(33)
Corporate Notes and Obligations
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	13,480
Less than 12 months, Unrealized Loss	(20)
U.S. Government Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	43,793
Less than 12 months, Unrealized Loss	$ (13)