Cash Deposits and Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Insurance collateral deposits	$ 3,767	$ 2,599
Excess loss fund	2,284	1,450
Accumulated interest income	814	812
State collateral deposits	57	143
Materials and supplies	273	354
Other	37	171
Total	$ 7,232	$ 5,529