Debt: (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended						3 Months Ended					3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	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	Mar. 31, 2013 Secured debt	Jun. 30, 2012 Secured debt	Jun. 30, 2012 Unsecured revolving credit facility	Mar. 31, 2013 Unsecured revolving credit facility	Mar. 31, 2013 Reef Shipping	Mar. 31, 2013 Matson South Pacific	Mar. 31, 2013 Matson South Pacific Reef Shipping	Mar. 31, 2013 Maximum Unsecured revolving credit facility
Debt
Maximum borrowing capacity									$ 375.0
Credit facility term									5 years
Current borrowing capacity										11.8
Cash borrowings										5.0
Letters of credit										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
Initial amount of the lease												2.9
Capital lease acquired											0.3
Capital leases													3.2
Capital leases classified as current													1.0
Total Debt	$ 301.1	$ 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%