Marketable Securities - Summary of Estimated Fair Value and Gross Unrealized Holding Losses of Marketable Securities in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
In Unrealized Loss Position For Less Than 12 Months, Fair Value	$ 27,952	$ 98,805
In Unrealized Loss Position For Less Than 12 Months, Gross Unrealized Losses	30	161
In Unrealized Loss Position For Greater Than 12 Months, Fair Value	4,671	0
In Unrealized Loss Position For Greater Than 12 Months, Gross Unrealized Losses	10	0
Municipal Notes and Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
In Unrealized Loss Position For Less Than 12 Months, Fair Value	27,952	98,805
In Unrealized Loss Position For Less Than 12 Months, Gross Unrealized Losses	30	161
In Unrealized Loss Position For Greater Than 12 Months, Fair Value	4,671	0
In Unrealized Loss Position For Greater Than 12 Months, Gross Unrealized Losses	$ 10	$ 0