DESCRIPTION OF THE BUSINESS (Details)	6 Months Ended
	Jun. 30, 2017 segment facility item
DESCRIPTION OF THE BUSINESS
Number of reportable segments | segment	2
Number of owned vessels in fleet | item	21
Number of chartered vessels | item	6
Ocean Transportation | SSAT
DESCRIPTION OF THE BUSINESS
Ownership interest in SSAT (as a percent)	35.00%
Number of terminal facilities on which SSAT provided terminal and stevedoring services on the U.S. West Coast | facility	6
MatNav | SSAT
DESCRIPTION OF THE BUSINESS
Number of terminal facilities on which SSAT provided terminal and stevedoring services on the U.S. West Coast | facility	3