The actual hull coverage ratio of the Company as at period end.

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The minimum liquidity required by covenants of certain debt currently held by the company. Liquidity is defined as cash, cash equivalents and undrawn committed revolving credit lines with at least six months to maturity.

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The minimum free liquidity and undrawn revolving credit line required by covenants of certain debt currently held by the Company, as a percentage of total debt.

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The minimum hull coverage ratio as required by certain covenants for debts held by the Company.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of lessee's undiscounted obligation for lease payments for finance lease.

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Amount of lessee's undiscounted obligation for lease payments in excess of discounted obligation for lease payments for finance lease.

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Weighted average discount rate for finance lease calculated at point in time.

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Amount of cash inflow before closing and debt issuance costs received by a seller-lessee in a sale-leaseback recognized in financing activities.

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A description of the terms of the lease(s) related to the assets being leased-back in connection with the transaction involving the sale of property to another party and the lease of the property back to the seller.

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Leases - Additional Information (Details)	1 Months Ended				9 Months Ended		12 Months Ended
Leases - Additional Information (Details)	May 31, 2019 USD ($) vessel	Nov. 30, 2018 USD ($) vessel	Sep. 30, 2018 USD ($) vessel	Jul. 31, 2017 USD ($) vessel	Sep. 30, 2019 USD ($) vessel	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) vessel
Finance Leased Assets [Line Items]
Proceeds from financing related to sales and leaseback of vessels \| $					$ 63,720,000	$ 156,644,000
Number Of Vessels					3		8
Finance Lease, Weighted Average Discount Rate, Percent					7.70%		7.50%
LR2 Tankers [Member]
Finance Leased Assets [Line Items]
Number Of Vessels					2
Suezmax, Aframax and LR2 Vessels [Member]
Finance Leased Assets [Line Items]
Finance Lease, Liability, Payment, Due \| $					$ 615,900,000		$ 557,100,000
Finance Lease, Liability, Undiscounted Excess Amount \| $					$ 195,000,000		$ 181,800,000
Aframax Tanker
Finance Leased Assets [Line Items]
Number Of Vessels Obligated to Purchase \| $					6
Suezmax Tankers
Finance Leased Assets [Line Items]
Number Of Vessels Obligated to Purchase \| $					2
Minimum [Member]
Finance Leased Assets [Line Items]
Sale Leaseback Transaction, Lease Terms					P9Y
Maximum
Finance Leased Assets [Line Items]
Sale Leaseback Transaction, Lease Terms					P12Y
Option to Purchase July 2020- Nov 2021[Member] \| Suezmax, Aframax and LR2 Vessels [Member]
Finance Leased Assets [Line Items]
Number Of Vessels					16
Finance Lease Obligations [Member]
Finance Leased Assets [Line Items]
Number of Lessors					16
Finance Lease Obligations [Member] \| Suezmax, Aframax and LR2 Vessels [Member]
Finance Leased Assets [Line Items]
Minimum liquidity covenant requirement \| $					$ 35,000,000.0
Minimum liquidity as a percentage of consolidated debt covenant requirement					5.00%
Finance Lease Obligations [Member] \| Minimum [Member] \| Suezmax, Aframax and LR2 Vessels [Member]
Finance Leased Assets [Line Items]
Debt Instrument, Term					6 months
July 2017 Sale Leaseback [Member] \| Suezmax Tankers
Finance Leased Assets [Line Items]
Debt Covenant Minimum Hull Coverage Ratio, Thereafter					100.00%
Actual Hull Coverage Ratio					121.00%		101.00%
July 2017 Sale Leaseback [Member] \| Finance Lease Obligations [Member]
Finance Leased Assets [Line Items]
Period Required to Maintain 90% Hull Coverage Ratio					three
July 2017 Sale Leaseback [Member] \| Finance Lease Obligations [Member] \| Suezmax Tankers
Finance Leased Assets [Line Items]
Proceeds from financing related to sales and leaseback of vessels \| $				$ 153,000,000
Number Of Vessels				4	4
September 2018 Sale Leaseback [Member] \| Aframax Tanker
Finance Leased Assets [Line Items]
Maintain 78% Hull Coverage Ratio for the Second Year					75.00%
Debt Covenant Minimum Hull Coverage Ratio, Thereafter					90.00%
Actual Hull Coverage Ratio					113.00%		91.00%
Debt Covenant Minimum Hull Coverage Ratio, Year 2					78.00%
September 2018 Sale Leaseback [Member] \| Finance Lease Obligations [Member]
Finance Leased Assets [Line Items]
Number Of Vessels			6
September 2018 Sale Leaseback [Member] \| Finance Lease Obligations [Member] \| Aframax Tanker
Finance Leased Assets [Line Items]
Proceeds from financing related to sales and leaseback of vessels \| $			$ 156,600,000
Number Of Vessels			6		6
November 2018 Sale leaseback Transaction [Member] \| Finance Lease Obligations [Member]
Finance Leased Assets [Line Items]
Number Of Vessels		4
November 2018 Sale leaseback Transaction [Member] \| Finance Lease Obligations [Member] \| LR2 Tankers [Member]
Finance Leased Assets [Line Items]
Number Of Vessels		1
November 2018 Sale leaseback Transaction [Member] \| Finance Lease Obligations [Member] \| Suezmax, Aframax and LR2 Vessels [Member]
Finance Leased Assets [Line Items]
Proceeds from financing related to sales and leaseback of vessels \| $		$ 84,700,000
Number Of Vessels		4			4
November 2018 Sale leaseback Transaction [Member] \| Finance Lease Obligations [Member] \| Aframax Tanker
Finance Leased Assets [Line Items]
Number Of Vessels		2
November 2018 Sale leaseback Transaction [Member] \| Finance Lease Obligations [Member] \| Suezmax Tankers
Finance Leased Assets [Line Items]
Number Of Vessels		1
May 2019 Sale Leaseback [Member] \| Suezmax Tankers
Finance Leased Assets [Line Items]
Maintain 78% Hull Coverage Ratio for the Second Year					75.00%
Debt Covenant Minimum Hull Coverage Ratio, Thereafter					90.00%
Actual Hull Coverage Ratio					108.00%		0.00%
Debt Covenant Minimum Hull Coverage Ratio, Year 2					78.00%
May 2019 Sale Leaseback [Member] \| Finance Lease Obligations [Member]
Finance Leased Assets [Line Items]
Number Of Vessels	2
May 2019 Sale Leaseback [Member] \| Finance Lease Obligations [Member] \| Suezmax Tankers
Finance Leased Assets [Line Items]
Proceeds from financing related to sales and leaseback of vessels \| $	$ 63,700,000
Number Of Vessels	2				2
Variable Interest Entity, Primary Beneficiary [Member]
Finance Leased Assets [Line Items]
Number of Lessors					14