Long-Term Debt - Revolving Credit Facilities - Additional Information (Details)	12 Months Ended
Dec. 31, 2019 USD ($) credit_facility Subsidiaries	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Reduction in 2020	$ 393,800,000
Reduction in 2021	412,000,000.0
Reduction in 2022	$ 298,500,000
Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	0.30%
Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.25%
Revolving Credit Facility
Debt Instrument [Line Items]
Borrowings provided under revolving credit facilities	$ 225,000,000
Reduction in 2020	23,400,000
Reduction in 2021	24,400,000
Reduction in 2022	$ 330,400,000
Line of Credit \| Revolving Credit Facility \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.40%
Line of Credit \| Revolving Credit Facility \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.25%
Unsecured Debt
Debt Instrument [Line Items]
Number of credit facilities \| credit_facility	1
Collateralized Mortgage Backed Securities
Debt Instrument [Line Items]
Number of vessels \| credit_facility	2
Number of subsidiaries \| Subsidiaries	2
Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Number of credit facilities \| credit_facility	2
Borrowings provided under revolving credit facilities	$ 378,200,000	$ 400,600,000
Undrawn amount of revolving credit facilities	$ 166,200,000	$ 175,600,000

Number Of Credit Facilities

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- Definition

Number Of Vessels

+ References

No definition available.

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- Definition

Number of Subsidiaries

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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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.

+ References

No definition available.

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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

+ References

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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