Marketable Securities - Summary of Estimated Fair Value and Gross Unrealized Holding Losses of Marketable Securities in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
In Unrealized Loss Position For Less Than 12 Months, Fair Value	$ 10,400	$ 27,952
In Unrealized Loss Position For Less Than 12 Months, Gross Unrealized Losses	1	30
In Unrealized Loss Position For Greater Than 12 Months, Fair Value	4,676	4,671
In Unrealized Loss Position For Greater Than 12 Months, Gross Unrealized Losses	4	10
Municipal Notes and Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
In Unrealized Loss Position For Less Than 12 Months, Fair Value	10,400	27,952
In Unrealized Loss Position For Less Than 12 Months, Gross Unrealized Losses	1	30
In Unrealized Loss Position For Greater Than 12 Months, Fair Value	4,676	4,671
In Unrealized Loss Position For Greater Than 12 Months, Gross Unrealized Losses	$ 4	$ 10