Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	$ 1,156	$ (862)	$ 518
Proceeds from available for sale municipal bonds	28,500	28,100	16,400
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized (loss) gain on sales of available-for-sale securities		$ 200
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized (loss) gain on sales of available-for-sale securities	$ (100)		$ 100