Long-Term Debt - Additional Information - Revolvers and Term Loans (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) credit_facility vessel	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 34,300,000	$ 32,000,000
Total principal	$ 382,313,000	$ 249,568,000
Debt Covenant Minimum Hull Coverage Ratio	100.00%
Minimum liquidity covenant requirement	$ 35,000,000
Minimum liquidity as a percentage of consolidated debt covenant requirement	5.00%
Weighted-average effective interest rate	2.50%	2.60%
Aggregate annual long-term principal repayments, remainder of 2021	$ 2,800,000
Aggregate annual long-term principal repayments, 2022	70,800,000
Aggregate annual long-term principal repayments, 2023	107,400,000
Aggregate annual long-term principal repayments, 2024	$ 201,300,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Number Of Debt Instruments \| credit_facility	1
Line of Credit Facility, Maximum Borrowing Capacity	$ 391,200,000	$ 438,400,000
Line of Credit Facility, Remaining Borrowing Capacity	65,000,000	253,400,000
Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Remaining Fiscal Year	44,200,000
Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Year One	80,400,000
Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Year Two	65,300,000
Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Year Three	201,300,000
Total principal	$ 326,167,000	185,000,000
Term Loans
Debt Instrument [Line Items]
Number Of Debt Instruments \| credit_facility	1
Total principal	$ 56,146,000	$ 64,568,000
Term Loan Due 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Collateral, Number of Vessels \| vessel	4
Debt Covenant Minimum Hull Coverage Ratio	125.00%
Actual Hull Coverage Ratio	177.00%
2020DebtFacilityMaturinginDecember2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Collateral, Number of Vessels \| vessel	30
Debt Covenant Minimum Hull Coverage Ratio	125.00%
Actual Hull Coverage Ratio	212.00%
Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Actual Hull Coverage Ratio	104.00%	121.00%
Long-term Debt, Term	6 months
London Interbank Offered Rate (LIBOR) [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.40%	2.40%
London Interbank Offered Rate (LIBOR) [Member] \| Term Loans
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%	2.25%

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

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- Definition

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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No definition available.

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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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Debt Instrument, Collateral, Number of Vessels

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Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Year One

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Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Remainder of Fiscal Year

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Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Year Three

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Long-term Debt, Revolving Credit Facility, Decrease in Amount Available Year Two

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Number of debt instruments.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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- Definition

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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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in remainder of current fiscal year.

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- Definition

Period between issuance and maturity of long-term debt, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

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