Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Prepaid expense and other assets	$ 7,224	$ 6,025
Prepaid licenses, permits and tolls
Variable Interest Entity [Line Items]
Prepaid expense and other assets	1,521	1,398
Prepaid Insurance
Variable Interest Entity [Line Items]
Prepaid expense and other assets	4,628	3,574
Other Current Assets
Variable Interest Entity [Line Items]
Prepaid expense and other assets	$ 1,075	$ 1,053