Debt: (Details) (USD $) In Millions, unless otherwise specified				3 Months Ended						3 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Matson South Pacific	Jun. 30, 2012 Unsecured debt item	Jun. 30, 2012 Unsecured debt, tranche maturing in 2023	Jun. 30, 2012 Unsecured debt, tranche maturing in 2027	Jun. 30, 2012 Unsecured debt, tranche maturing in 2032	Jun. 30, 2013 Secured debt	Jun. 30, 2012 Secured debt	Jun. 30, 2012 Unsecured revolving credit facility	Jun. 30, 2013 Unsecured revolving credit facility	Jun. 30, 2013 Unsecured revolving credit facility Maximum	Jun. 30, 2013 Unsecured revolving credit facility Minimum
Debt
Maximum borrowing capacity										$ 375.0
Credit facility term										5 years
Used portion of credit facility											6.8
Debt issued				170.0	77.5	55.0	37.5
Number of tranches				3
Interest rate (as a percent)					3.66%	4.16%	4.31%
Weighted average interest rate (as a percent)				3.97%
Weighted average period				9 years 3 months 18 days
Release of MV Manulani as security for debt									56.0
Secured Debt								70.4
Capital leases			3.0
Total Debt	$ 292.4	$ 319.1
Debt covenant, required ratio of debt to consolidated EBITDA												3.25
Ratio of consolidated EBITDA to interest expense													3.50
Debt covenant, required principal amount of priority debt as a percentage of consolidated tangible assets												20.00%
Debt covenant, required principal amount of priority debt (excluding Title XI) as a percentage of consolidated tangible assets												10.00%