LONG-TERM DEBT AND FINANCING ARRANGEMENTS (Assets Sec) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Arrangements
Total assets securing notes payable or held under capital leases	$ 222,367	$ 138,492
Less accumulated depreciation and amortization	61,643	25,120
Net assets securing notes payable or held under capital leases	$ 160,724	113,372
Weighted-average interest rate (as a percent)	2.30%
Interest paid, net of capitalized interest	$ 4,500	4,000	$ 2,700
Capitalized interest	700	200	$ 100
Revenue equipment
Financing Arrangements
Total assets securing notes payable or held under capital leases	220,566	136,698
Land and structures (terminals)
Financing Arrangements
Total assets securing notes payable or held under capital leases	1,794	$ 1,794
Other equipment
Financing Arrangements
Total assets securing notes payable or held under capital leases	$ 7