Fair Value Measurements - Principal Amount And Estimated Fair Value Of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	$ 313,850	$ 404,732
Fair value	326,897	418,431
Term Loan Repaid September 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount		29,750
Fair value		28,969
Nordea Q5000 Loan Matured 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	48,850	56,410
Fair value	53,115	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	34,638	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	29,001	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	$ 210,143	$ 211,383