Marketable Securities - Summary of Estimated Fair Value and Gross Unrealized Holding Losses of Marketable Securities in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position For Less Than 12 Months, Fair Value	$ 50,097	$ 98,805
Unrealized Loss Position For Less Than 12 Months, Gross Unrealized Losses	(70)	(161)
Unrealized Loss Position For Greater Than 12 Months, Fair Value	7,172	0
Unrealized Loss Position For Greater Than 12 Months, Gross Unrealized Losses	(17)	0
Municipal Notes and Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position For Less Than 12 Months, Fair Value	50,097	98,805
Unrealized Loss Position For Less Than 12 Months, Gross Unrealized Losses	(70)	(161)
Unrealized Loss Position For Greater Than 12 Months, Fair Value	7,172	0
Unrealized Loss Position For Greater Than 12 Months, Gross Unrealized Losses	$ (17)	$ 0