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 | |