Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows From Operating Activities:
Net income	$ 193.1	$ 82.7	$ 109.0
Reconciling adjustments:
Depreciation and amortization	114.9	100.4	94.4
Amortization of operating lease right of use assets	74.8	60.7
Deferred income taxes	52.1	23.6	29.3
Loss (Gain) on disposal of property and equipment	2.8	(1.4)	(1.9)
Share-based compensation expense	18.8	11.3	12.1
Income from SSAT	(26.3)	(20.8)	(36.8)
Distributions from SSAT	55.4	25.2	42.0
Changes in assets and liabilities:
Accounts receivable, net	(48.0)	17.8	(29.1)
Deferred dry-docking payments	(16.8)	(25.9)	(19.2)
Deferred dry-docking amortization	25.1	34.3	37.4
Prepaid expenses and other assets	21.9	24.5	4.2
Accounts payable, accruals and other liabilities	44.8	(13.9)	71.2
Operating lease liabilities	(75.9)	(59.9)
Other long-term liabilities	(6.9)	(9.8)	(7.6)
Net cash provided by operating activities	429.8	248.8	305.0
Cash Flows From Investing Activities:
Capitalized vessel construction expenditure	(87.8)	(219.1)	(338.6)
Other capital expenditures	(104.5)	(91.2)	(62.6)
Proceeds from disposal of property and equipment	15.3	3.4	136.3
Cash deposits into Capital Construction Fund	(132.4)	(96.2)	(340.0)
Withdrawals from Capital Construction Fund	132.4	96.2	340.9
Proceeds from sale of other investments			3.7
Net cash used in investing activities	(177.0)	(306.9)	(260.3)
Cash Flows From Financing Activities:
Proceeds from issuance of debt	325.5
Repayments of debt	(216.5)	(42.1)	(30.7)
Proceeds from revolving credit facility	648.0	622.1	963.9
Repayments of revolving credit facility	(955.3)	(478.0)	(933.9)
Payment of financing costs	(18.5)
Proceeds from issuance of common stock	0.1	0.3	0.7
Dividends paid	(39.2)	(37.2)	(35.4)
Tax withholding related to net share settlements of restricted stock units	(5.6)	(3.1)	(4.6)
Net cash provided by (used in) financing activities	(261.5)	62.0	(40.0)
Net (Decrease) Increase in Cash, Cash Equivalents and Restricted Cash	(8.7)	3.9	4.7
Cash, Cash Equivalents and Restricted Cash, Beginning of the Year	28.4	24.5	19.8
Cash, Cash Equivalents and Restricted Cash, End of the Year	19.7	28.4	24.5
Reconciliation of Cash, Cash Equivalents, and Restricted Cash, at End of the Year:
Total Cash, Cash Equivalents and Restricted Cash, End of the Year	28.4	24.5	24.5
Supplemental Cash Flow Information:
Interest paid, net of capitalized interest	26.2	22.0	18.3
Income tax paid, net of income tax refunds	(16.1)	(24.2)	5.2
Non-cash Information:
Capital expenditures included in accounts payable, accruals and other liabilities	$ 24.7	$ 8.5	$ 4.1