Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Contract with Customer, Asset, Net [Abstract]
Contract with Customer, Asset, Net	$ 1,100								$ 1,100
OPERATING REVENUE	$ 147,003	$ 151,279	$ 155,826	$ 156,695	$ 165,703	$ 182,114	$ 129,616	$ 129,903	610,803	$ 607,336	$ 612,937
Fuel surcharge revenue									85,300	72,500	58,400
Revenue from Contract with Customer, Including Assessed Tax									$ 14,900	$ 24,300	$ 29,400