Debt: (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended	9 Months Ended			3 Months Ended	1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility Minimum	Sep. 30, 2012 Unsecured revolving credit facility Maximum [Member]	Jun. 30, 2012 Unsecured Debt [Member] 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, 2012 Secured debt	Sep. 30, 2012 Secured debt MatNav
Debt
Maximum borrowing capacity			$ 375.0
Credit facility term			5 years
Current borrowing capacity				37.0
Cash borrowings				30.0
Letters of credit				7.0
New unsecured, fixed rate, amortizing long-term debt							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	72.6
Total debt	$ 328.6	$ 197.5
Debt covenant, required ratio of debt to consolidated EBITDA				1.99		3.25
Ratio of consolidated EBITDA to interest expense				16.71	3.50
Debt covenant, required principal amount of priority debt as a percentage of consolidated tangible assets				6.50%		20.00%
Debt covenant, required principal amount of priority debt (excluding Title XI) as a percentage of consolidated tangible assets						10.00%