Fair Value Measurements - Principal Amount And Estimated Fair Value Of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 28, 2019	Dec. 31, 2018	Mar. 20, 2018	Nov. 01, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	$ 445,949
Term Loan Maturing June 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	0		$ 33,693
Fair value	0		33,314
Term Loan Maturing December 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	34,125	$ 35,000	0
Fair value	33,698		0
Nordea Q5000 Loan Maturing April 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	98,214		125,000
Fair value	98,214		122,500
MARAD Debt Maturing February 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	63,610		70,468
Fair value	68,972		74,406
Convertible Senior Notes Maturing May 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	125,000		125,000		$ 125,000
Fair value	126,094		114,298
Convertible Senior Notes Maturing September 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	125,000		125,000	$ 125,000
Fair value	146,719		114,688
Total Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	445,949		479,161
Fair value	$ 473,697		$ 459,206