Cash Deposits and Other Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Insurance collateral deposits	$ 3,517	$ 3,767
Excess loss fund	1,959	2,284
Accumulated interest income	627	814
State collateral deposits	53	57
Materials and supplies	331	273
Other	36	37
Total	$ 6,523	$ 7,232