Property and Equipment - Components of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Aircraft	$ 19,260	$ 19,160
Equipment	20,930	19,788
Furniture and fixtures	7,639	7,201
Leasehold improvements	22,847	21,612
Property, plant and equipment, gross	70,676	67,761
Less accumulated depreciation and amortization	(62,074)	(58,997)
Total property and equipment, net	$ 8,602	$ 8,764