Fair Value Measurements - Principal Amount And Estimated Fair Value Of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 20, 2018	Dec. 31, 2017	Jun. 30, 2017	Nov. 01, 2016	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	$ 512,400
Term Loan Maturing June 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	36,032		$ 97,500	$ 100,000
Fair value	36,437		98,231
Nordea Q5000 Loan Maturing April 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	151,785		160,714
Fair value	152,164		160,111
MARAD Debt Maturing February 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	73,774		77,000
Fair value	80,135		82,058
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	119,531		124,219
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	0
Fair value	126,094		0
Convertible Senior Notes Maturing March 2032
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	809		60,115			$ 200,000
Fair value	809		60,040
Loan Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	512,400		520,329
Fair value	$ 515,170		$ 524,659