Investments and Fair Value Measurements - Gross Unrealized Losses and Fair Values of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 69,910	$ 63,631
Gross Unrealized Losses	(90)	(91)
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	8,101	10,162
Gross Unrealized Losses	(2)	(14)
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value	61,809	53,222
Gross Unrealized Losses	(88)	(77)
Municipal notes and bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	247
Gross Unrealized Losses	$ 0	$ 0