Fair Value Measurements - Principal Amount And Estimated Fair Value Of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	$ 345,676	$ 404,732
Fair value	396,657	418,431
Term Loan Maturing December 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	28,000	29,750
Fair value	27,895	28,969
Nordea Q5000 Loan Maturing January 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount		53,572
Fair value		53,598
MARAD Debt Maturing February 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	52,676	56,410
Fair value	56,542	62,318
Convertible Senior Notes Maturing May 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	35,000	35,000
Fair value	35,199	33,513
Convertible Senior Notes Maturing September 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	30,000	30,000
Fair value	29,662	28,650
Convertible Senior Notes Maturing February 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	200,000	200,000
Fair value	$ 247,359	$ 211,383