DESCRIPTION OF THE BUSINESS (Details)	12 Months Ended
	Dec. 31, 2017 segment facility
DESCRIPTION OF THE BUSINESS
Number of reportable segments | segment	2
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	7
MatNav | Ocean Transportation | SSAT
DESCRIPTION OF THE BUSINESS
Number of terminal facilities on which SSAT provided terminal and stevedoring services on the U.S. West Coast	4