Investments - Gross Unrealized Losses and Fair Value for Investments in Unrealized Loss Position (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	$ 43,288
Less than 12 months, Unrealized Loss	(74)
12 months or greater, Fair Value	35,464
12 months or greater, Unrealized Loss	(36)
Corporate Notes and Obligations
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	18,873
Less than 12 months, Unrealized Loss	(22)
12 months or greater, Fair Value	3,998
12 months or greater, Unrealized Loss	(3)
U.S. Government Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	24,415
Less than 12 months, Unrealized Loss	(52)
12 months or greater, Fair Value	31,466
12 months or greater, Unrealized Loss	$ (33)