PROPERTY AND EQUIPMENT - SUMMARY (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property and equipment
Cost	$ 2,444.2	$ 2,151.9
Accumulated depreciation of property and equipment	1,278.5	1,202.7
Property and equipment, net	1,165.7	949.2
Depreciation Expense	86.7	86.0	$ 76.4
Property and equipment subject to capital leases	4.9	6.4
Cost for construction of containerships	924.5
Capitalized interest	7.5	2.1	0.4
Net of accumulated depreciation	3.8	2.4
Amortization expenses	$ 1.5	1.2	$ 0.6
Number of vesels under agreements for contstruction | item	4
Vessels
Property and equipment
Cost	$ 1,433.6	1,416.1
Accumulated depreciation of property and equipment	893.2	840.7
Property and equipment, net	540.4	575.4
Containers and equipment
Property and equipment
Cost	543.0	536.9
Accumulated depreciation of property and equipment	349.0	326.7
Property and equipment, net	194.0	210.2
Terminal facilities and other property
Property and equipment
Cost	64.8	43.2
Accumulated depreciation of property and equipment	36.3	35.3
Property and equipment, net	28.5	7.9
Vessel construction in progress
Property and equipment
Cost	376.6	124.5
Property and equipment, net	376.6	124.5
Capitalized interest	10.4	2.9
Other construction in progress
Property and equipment
Cost	26.2	31.2
Property and equipment, net	$ 26.2	$ 31.2